A lot of historians used to take Vegetius' words at face value. Popular history such as you read still does, at times. This is a mistake. Vegetius is a moralising, antiquarian writer, who had no military experience whatsoever and is working from sources that by his time were already centuries old. He's seeking to instruct his contemporaries about a supposedly more virtuous and noble past, but his work is a hodgepodge of anachronisms and idealisations and often is quite simply wrong. It's a very interesting book, and had great influence over the centuries, but you have to be very careful when using it as a source for the period it ostensibly describes.

Unlike what Vegetius tells us, other evidence clearly demonstrates that the Roman armies of late antiquity continued to wear armour, continued to be well trained and disciplined, and continued to be an effective force. The Romans continue to win most of their battles until well past Vegetius' time.

There is some disagreement between modern historians on the extent of this: some say the army of late antiquity was as good or better as that of the principate, as well as being more numerous. Others say that a decline did take place in the 5th century. However, they blame this on the state losing control of its resources and/or being under considerably more outside pressure and thus being unable to afford quite the same standards in equipping and training their troops, rather than any lack of "Roman culture" or them "becoming soft."

As for your actual question: that is a very broad one. Answering it really depends on how you want to explain the fall of the Roman Empire in the first place. And there are quite a few reasons people have advanced to explain Rome's fall. (Nota bene: most of the listed reasons are nonsense, and many others are outdated. But it's still fun to read just how many things people have claimed explained it, even if most of them are wrong and many others unprovable.)

Obviously, if you think the Roman Empire fell because of long-term structural changes in the world of late antiquity, there will be fewer concrete decisions to point to than when you think the Empire fell because it had a string of weak emperors making mistakes at a critical time.

So, first you might want to read this series of posts and the ensuing discussion, in which I try to explain various theories historians advance to explain Rome's fall.

There's also the very important question of just who is making these decisions. Your question implies (but does not state outright) that it would be the emperors or senior military commanders making the decisions. But what if it the collapse of Rome happened on a more local level, with many local elites and peoples in the towns and villages of the Roman provinces deciding that they were better off siding with the so-called Barbarians, because the Roman empire was no longer defending their interestsOr what if the main cause was that the Germanic peoples to the north, thanks to their centuries-long contact with Rome, evolved and developed to the point where the Romans simply could not maintain the level of dominance they needed to keep their polity together? Then there isn't really any individual or group of individuals you can point to.

Still, even if many of the underlying factors (demographics, economics, cultural development) were out of the emperors' control, they did make plenty of policy decisions, some of which will have had long-term consequences. So we can examine those. In the broad sense, I find Goldsworthy's argument (explained in that thread) quite appealing: that Roman emperors took many decisions meant to improve the stability and security of their reign in the short term, but compromised the Empire's internal coherence and safety in the long term. For example: the practice of keeping large field armies with the emperors made them much safer from usurpation and reduced the risk of civil war, but meant that the people living in the frontier were less well defended, increasing disaffection in the long run. But these actions were not mistakes as such: they were reasonable responses to the challenges of the time.

There are also plenty of concrete Roman mistakes you might examine, but while it's easy to point to, say, Valen's mistakes at the battle of Adrianople leading to the loss of that battle, it's much harder to determine whether those kinds of events fundamentally weakened the empire in the long run, or were blips the Romans could simply absorb and recover from over time.

I can tell you one very concrete decision made by one man (and his advisors) that likely helped save the eastern half of the Roman empire for another millennium: the founding of Constantinople by the emperor Constantine. Without that city acting as a cultural, economic and political anchor, it does not seem likely the east would have held together in the face of the many challenges it encountered, particularly the Arab invasions. And if Constantine hadn't built his new capital in that exact, superbly defensible place, it is equally unlikely they could have protected their all-important centre for all those centuries. Constantine could not have predicted his decision would have had such consequences, of course, (nor could he have predicted what the consequences of his support for Christianity would be) so I'm not sure he should be getting that much credit, and the famous walls that protected the city were only build under a later emperor, but still. Constantine did know how to pick them.

I can also mention one very concrete decision made by one other man that did much to finally destroy the Roman way of life in Italy: The decision of the (eastern Roman) emperor Justinian to invade the place in the 6th century. The "Ostrogothic kingdom" ruling there at the time was very much Roman in outlook, culture and ambition, with its leaders claiming Roman titles for themselves and paying homage to the emperor in Constantinople, at least in name, and for the vast majority of Italy's residents their life continued pretty much the same as it had for all the centuries of Roman rule. Until the eastern Roman invasion and the ensuing 20 years of war reduced the region to a wasteland, that was then easy prey for the invading Longobards. This then is what finally killed Rome. But of course, you'll only find that view convincing if you agree with those historians who see the "Germanic" successor kingdoms of the 5th and 6th century as fundamentally Roman in nature. But even if you don't hold that view, Justinian's campaigns proved to be bad for the eastern Roman Empire as well, overstretching its resources and resulting in new territories that proved very difficult to defend.

I can't give you an answer, but I think that it is important to understand the timescales being discussed. The Roman empire did experience a period of decline that led to the collapse and formal end of the Western Roman Empire. However the East would stand for another millennium. Events in the following centuries would see periods of resurgence and decline, before the final collapse and destruction in 1453.

It is perhaps not my place to do so, but can I ask which period you are referring to, if you wish to specify at all?

Follow up Question: How significant were The Crisis of the Third Century and the wars of the tetrarchy in this process for the WRE? Were there any vulnerabilities that had emerged and were not accounted for?