My weekdays are incredibly hectic, so I apologize if this answer seems a bit rushed. Please feel free to ask any questions, I will address them later as best I can.

As during the Principate, the bulk of Roman-born recruits were peasants, young men from agricultural families, either working their own land or, more likely, working the estates of a larger landowner. Recruitment was not evenly spaced throughout the Empire; men from poorer, more rural provinces (like Pannonia, Isauria, and Illyria) tended to provide the most recruits. Most recruits were not of urban extraction, although we have little data in this regard. While all units recruited from their locality, recruitment as a process was overseen by the regional Praetorian Prefectures.

Just as in the Principate, young men were attracted to military life due to the many attractions it offered. Although the value of military pay in Late Antiquity is debated, having even a little spending money was likely a new experience for men from families used to subsistence living. While annual salaries may (or may not) have been poor, donatives, a bonus paid upon the accession of an emperor, or to commemorate it, were of far greater value. Accession donatives were five gold solidi and one pound of silver to all ranks, while quinquennial donatives were five solidi. If we consider that five solidi was roughly the average annual income of a Roman peasant family, this is not an insignificant sum. Soldiers also received their rations and equipment free from the state; in the Principate, these had been paid for by salary deductions. While the details of the Late Antique military medical service remain unclear, we know qualified care continued. Soldiers thus had access to comprehensive medical care, something they doubtless lacked in civilian life. Upon honorable discharge, veterans received a discharge bounty, and they and their families were exempt from the poll tax. When taken together, military service continued to have many tangible benefits to attract recruits.

We can therefore look upon Vegetius’ claim that young men were supposedly unwilling to serve with a large degree of skepticism. Vegetius himself is a quite problematic source, as his work is not intended to be an accurate summary of the army. Rather, it is, to quote Michael Whitby, a “selectively critical epitome”. In the great moralizing tradition of Greco-Roman literature, Vegetius uses descriptions of the army of the Principate to comment on supposed contemporary military deficiencies. Yet Vegetius was a bureaucrat, not a soldier- he had no practical military experience. Furthermore, by comparining a “declined” present to a “morally superior” past, Vegetius is reciting a common trope in Greco-Roman literature. While Vegetius isn’t useless as a source, his work must be realized for what it is- a work of elite literature, written within the conventions of elite Greco-Roman historical writing. It is not, nor was intended to be, an accurate commentary on military affairs (unlike, say, the Strategikon).