If I could ask a more general follow up question: Was the Roman military of late antiquity in the whole a less professional, less capable fighting force than during the Principate?

Vegetius is a problematic source to begin with and I wouldn't take De Re Militari too seriously. Vegetius wrote in the late fourth century AD, and was likely a veterinarian with no military experience. De Re Militari is considered an emotional plea for a return to the military structures of the glorious past, rather than a serious attempt at aiding contemporary generals. To give one example, he mentions the long outdated manipular army, obsolete as of 107 BC with the reforms of Marius, rather than the Late Republican and Imperial Legions organized by cohort. Another flaw in Vegetius' logic is that the equipment of the Hastati, Principes and Triarii is covered in detail by Polybius, and only the wealthier soldiers used a cuirass at all. The rest of the soldiers wear a square piece of brass on their chests called a pectorale which would hardly be the difference between victory and defeat in battle. The following quote by the military historian Charles Oman encapsulates the disdain of Vegetius' De Re Militari within the historical community,

Vegetius was a theoretical admirer of the old legion, and wholly destitute of any insight into the meaning of the changes in military science that had taken place during the last hundred years

It's quite rare to find reference to armor at all in antique, late antique, and early medieval sources. When armor is mentioned it is generally in passing and not offered as a significant factor in an army's fighting ability or the outcome of a battle. There are certainly exceptions, descriptions of the armor of Byzantine heavy cavalry in the 10th century is painstakingly detailed, and there are accounts of battles where the cavalry were able to push through dense formations of infantry, typically a death sentence for lighter armed cavalry. For infantry equipment, the later Byzantine sources, well versed in fighting the horse archers of the scythians, arabs, avars, and so on, place emphasis on the shields of their infantry rather than armor. Going as far as describing the (rather large) minimum dimensions a shield should be whereas light armor of padded felt and linen was all that was required. Armor was good to have when it could be acquired, but the sources put far more emphasis or morale, generalship, tactics, and divine intervention.

When Vegetius was writing, we can be sure the memory of the crushing defeat at Adrianople in 378 was fresh in his mind. Three Roman armies were destroyed and the Emperor killed. This became a problem on a strategic level as the primary field forces of the Eastern Empire had been annihilated. However, it seems that Vegetius had a poor idea of what happened at the battle. Our best source comes from Ammianus Marcellinus, who attributes the defeat to a few factors, none of them being the equipment of the Roman army. First off, the Emperor Valentinian pushed forward to attack after a long march, rather than letting his exhausted army camp and recover in safety. Secondly, the Roman cavalry gave a pitiful performance. They seem to have been routed and never seen again almost as soon as the Gothic cavalry arrived on the battlefield, leaving the infantry formation unprotected and vulnerable to envelopment. As I have mentioned before, Adrianople is one more example where generalship and morale are the key indicators of success or defeat, rather than equipment.

That being said, there are two interesting passages in Ammianus' account that are relevant to our discussion.

First,

"And while arms and missiles of all kinds were meeting in fierce conflict," … "to inflict disaster on the Romans, our men began to retreat; but presently, roused by the reproaches of their officers, they made a fresh stand, and the battle increased like a conflagration, terrifying our soldiers, numbers of whom were pierced by strokes from the javelins hurled at them, and from arrows."

Two interesting points here, arrows and missiles were successful enough against the Roman Infantry formation to be described at all, but not successful enough to prevent the Romans from forming up again.

Second,

with mutual blows of battle-axes, helmets and breastplates were dashed in pieces.

This line could simply be adding some color to the disaster, but Ammianus is describing Roman helmets and breastplates so it's possible the Romans were wearing their armor.

As far as protecting oneself against the Hunnic bow, there seems to be a sort of legendary connotation to the weapon that is not warranted in my opinion. Composite bows of the Hunnic style were extremely difficult to make, taking months to complete. It is extremely unlikely that the Alans or Goths were using them. They also weren't magic, rendering shields and armor obsolete one thousand years before gunpowder. As a general rule Hunnic composite bows were as powerful as more common self bows (made from a single piece of wood) but much smaller and lighter, making them ideal for use from the saddle. Harking back to my earlier example of the Byzantines in the tenth and eleventh centuries, a large sturdy shield and a spear were enough to keep yourself safe from horse archers. A practical example of this is the battle of Manzikert, where the Emperor and general Romanos IV keep his infantry formation together through a grueling day of battle against an almost entirely mounted force of Turkish cavalry, many of which armed with bows. It wasn't the horse archers that broke down the Roman infantry formation, but desertions and a false rumor that Romanos had died.

Now that we've dismantled Vegetius' claims we can move on to what actually put the Romans in such dire straits by the time Vegetius put quill to parchment (and a little beyond). This is an intensely complex topic that has been studied and analyzed ad nauseum. However, I'll do my best to give a few of the broad strokes.

1. Crisis of the Third century. The inability of the Roman Empire to establish clear succession rules devolved into almost 100 years of instability and civil war. The 3rd century crisis wrecked the economy causing rampant inflation rendering the wages paid to soldiers and others worthless I will rephrase, inflation was eventually solved for the most part by Constantine's introduction of the Solidus. However, the Empire still had trouble finding Roman volunteers for the army, in part because wages were very low. It would not be until the Emperor Anastasius raised pay considerably, that domestic Roman recruits would form the majority of the field armies. It also resulted in the loss of tens of thousands of Roman soldiers if not more. These losses in men, material, and just as importantly, experience and esprit de corps, would never really be recovered.

2. Increase in the required activity of the Roman military. The 3rd century crisis consistently pulled soldiers away from the border territories to fight Roman opponents in constant civil wars. Lack of soldiers in border territories meant fewer expeditions to show Roman strength across the border, and fewer punitive strikes to punish incursions into Roman lands. This created a sort of positive feedback loop where more and more raids were launched into Roman lands, and these raids were more and more successful and less frequently punished.

3. Rise of Sassanid Persia. Roman-Persian relations are incredibly complex and too lengthy for me to get into here, but the Romans were always aware of the eastern threat which kept large armies in the east even during peace time. Julian's catastrophic campaign against Persia is another lowlight.

4. The Great Migrations. Third and Fourth century AD Hunnic migrations pushed large numbers of people into contact with Rome. The best known examples being the Visigoths and Ostrogoths. While the Romans are partly at fault for many poor decisions, Gothic movement into the empire resulted in several negative consequences. Such as the defeat at Adrianople, the Visigothic sack of Rome in 410, destabilization of Roman political structures as Gothic generals supported or revolted against various Emperors.

   Bonus point: the Crossing of the Rhine in 406 by the Vandals, Suebi, and Alans. It's unclear if we should attribute this to 2.) or 4.) but the end result was decades of conflict, Rome sacked again, and the loss of the African provinces which spelled certain doom for the Western Roman Empire.

All of these factors combined to create a situation where the Empire was is desperate need of soldiers, however, the money they had to pay the soldiers made almost worthless by rampant inflation, when soldiers could be found (mostly conscripted) they were less effective than in the past. This created another negative feedback loop, where the Romans increasingly lost control over territory, which sapped the tax and manpower base, which made if more difficult to protect their remaining territory and so on.

Edit 1 for spelling

Edit 2 based on feedback from /u/FlavivsAetivs

This is an interesting question - I am not a Roman military historian so forgive me if this isn’t specific enough; I am however currently studying the fall of the Western Roman Empire, so I’ll attempt an answer both to this and the followup question below - in one post because there is a nice overlap where certain contemporary arguments are relevant to both questions.

Contemporary historians’ exact thoughts on the merits of armour - or rather its supposed discontinued use - I couldn’t find, but I suspect it was not perceived as a major issue, as they do stress other factors that led to the weakening of the army. In terms of military success, the focus seemed to have been on more general factors such as discipline, tactics, and luck. In wider terms, internal weaknesses (such as taxation and corruption) and the influx of barbarians - not just Goths and Alans but a myriad of others - were also seen as detrimental to the strength not only of the army but the Empire as a polity. The latter two factors are important in the modern discussion around the strength of the Empire, and shed light on the follow up question.

Before talking about those two factors, however, here are some contemporary views related to the Roman military. Describing a battle between Roman forces and the Alamanni, Ammianus Marcellinus wrote that “the savages, thrown into disorder by their violence and anger, flamed up like fire, and hacked with repeated strokes of their swords at the close-jointed array of shields, which protected our men like a tortoise-formation … in a way the combatants were evenly matched, the Alamanni were stronger and taller, our soldiers disciplined by long practice.” [XVI., 12, 43-48]

Of course, AM is talking about the period directly preceding Gratian; nevertheless, it seems interesting that when comparing the two forces he makes no mention of armour as the differentiating factor, instead discussing general training and discipline. Indeed, for many contemporaries the fundamental difference between a Roman and a barbarian (i.e. an ‘uncivilised’ person - the term was used liberally and was highly subjective) was the exercise of discipline, not only on the battlefield but in following laws and exerting self control.

As an interesting aside, from AM’s accounts it seems that armour was already changing before the time of Gratian. AM describes a “fiery band of nobles” who, “being intent upon avoiding wounds … protected themselves like murmillos” [XVI., 12, 48-51]. This is a reference to the ‘murmillones’, a type of gladiator who’s armour was Gallic in fashion. AM also describes a procession into Rome, where “twin lines of infantrymen … clad in shining mail” marched alongside “the full-armoured cavalry … thin circles of iron plates, fitted to the curves of their bodies, completely covered their limbs” [XVI., 10, 8-12]. Clearly developments in the types of armour being worn had occurred over the course of the empire - this suggests that perhaps the retainment of the classic ‘legionnaire’ armour was not as important as Vegetius implies.

Indeed, his assertion that armour was abandoned at all is dubious, at least when reading Zosimus’ ‘Historia Nova’. In his account of a battle between emperor Theodosius I (successor of Gratian) and various barbarian tribes, Zosimus writes that the emperor, having learnt that the enemy had “intoxicated themselves” commanded his soldiers to “take with them only their swords and bucklers, and not their heavy armour as usual … this they performed, and destroyed in a very short space of time all the Barbarians.” This account suggests two things about the use of armour. First, in order to take off ‘heavy armour’, the infantrymen must have had that armour to begin with, and thus not ‘lazily abandoned’ it. Secondly, it stresses the use of discipline - or the barbarian’s lack thereof - and tactics - the Roman response - instead of armour.

Such factors can even seem strange to modern eyes. For example, divine intervention or luck was often highlighted instead of more practical features (such as armour). In his vivid account of the unsuccessful attempt on Constantinople by a joint force of Goths and Huns, AM asserts that one major influence leading to their failure was the actions of an Oriental man “naked except for a loin-cloth, uttering hoarse and dismal cries”, drew his dagger and “rushed into the thick of the Gothic army, and after killing a man applied his lips to his throat and sucked the blood that poured out. The barbarians, terrified by this strange and monstrous sight, after that did not show their usual self-confidence when they attempted any action, but advanced with hesitating steps.” Indeed the very presence of that ‘Oriental man’ was due to luck - or as AM put it, “celestial power” - for the Saracen army of which he was a member happened to arrive at the city at the same time as the besieging forces. [XXXI., 16, 3-5]

Thus, it seems clear that, in the eyes of contemporary historians, armour was not a deciding factor in deciding military success. Granted, they do not directly comment on changing policies surrounding armour; from what they have said, however, it can most probably be safely inferred that other factors were seen as more important: luck (or rather divine intervention), tactics, and discipline were all stressed as crucial where armour was not. Interestingly, not only does it appear that armour styles had changed before the time of Gratian, it seems that armour was still widely used in the army after his reign, calling Vegetius’ assertion itself into question.

Contemporary historians also commented on more general changes that were perceived to have weakened the Western Roman Army. These were internal weaknesses - such as corruption and taxation - and the policy of attempting to incorporate the vast influx of barbarian tribes into the empire and army as federates and mercenaries. Again, their prominence in contemporary thought - rather than the merits of armour - suggests the relative unimportance of the latter factor in their minds. These broader factors also feature prominently in modern historical narratives not only surrounding the army but the fall of the Western Empire.

Halsall argues that there was a fundamental shift in attitudes amongst Roman elites from the first to the fourth century. Rather than sponsoring public works, people focused on exploitation and private profit. Indeed, in 444 legislation was implemented that regularised the practise of paying for desirable public positions. This attitude unsurprisingly lead to a corrupt army. Libanius, in a c.390 address to the emperor, wrote that soldiers in rural billets were “all on the take” and selling protection - “so then, the curiae are hurt by this fine racket, and the cities are hurt by the harm done to their curiae, and, too, the fighting forces are hurt because of the harm done to these latter.” Widespread state corruption on a larger scale than before weakened not only the empire but its armed forces.

Taxation was also a problem for the Western Roman Empire. Its army undoubtably required vast sums of money to maintain; Wickham estimates that it required around 1/3 of imperial tax revenue was to fund. The tax situation declined drastically over the 4th and 5th centuries: loss of territories, such as North Africa (the wealthiest) in 435; increasingly frequent barbarian raids; bloody civil wars; barbarian ‘kings’ and regional elites declaring themselves independent of the empire; Roman leaders gifting barbarian tribes lands and their associated tax revenues; all these factors increased the stress on the imperial economic machine. Indeed, there are tantalising hints at the frustration felt by Roman rulers: 5th century imperial legislation openly admitted that plans for a larger army were being frustrated by the inability to feed or clothe the current one.

The other key factor seen as weakening the Roman army was the increased use of barbarians as troops. Ammianus Marcellinus viciously attacks the “stormy eagerness” of Roman leaders who allowed barbarians to settle in the empire as federates; “on the part of [such] insistent men was the ruin of the Roman world brought in.” [XXXI., 4, 6]. Probably motivated by aforementioned economic problems - the settled barbarians would provide troops, allowing tax collectors to take much needed gold from provinces instead of conscripts - it was short-sighted at best; consider the contemporary views surrounding barbarian troops - their replacement of disciplined Roman troops seems a weakness. Furthermore, their loyalty was questionable at best, and they frequently rebelled or simply disobeyed orders; for example, according to Hydatius’ account, Goths sent to Spain to subdue the Suevi instead ransacked the North of the country - including loyal Hispano-Romans - for plunder. In the 5th century especially the military situation increasingly degenerated into a convoluted system of ‘alliances’, with Romans, Huns, and Barbarians fighting with and against the others at various points. This reliance on non-Roman troops strongly suggests that the imperial army as an institution was far weaker than in previous centuries.

It is clear that through internal and external factors the Roman empire was significantly weakened by the 5th century. Indeed, Gallic chronicles “make it clear that the government in Ravenna (capital of the Western Empire from 402) was in no position to do much more than to play one set of barbarians against another.” [Cameron]. As the taxation situation spiralled downwards, weighed down by corruption and exacerbated by those tribes - both in their plundering and settlement - the Roman army became a less capable institution. If the reliance on barbarian troops is not enough to show that, the inability of the empire to hold onto even its richest provinces suggests a fundamental weakness in Roman armed forces.