I'm getting closer to being ready, or as ready as I'll ever be, to write my Celtic Classics Conference paper for late July. Whatever I don't get around for the conference, I'll be sure to get to when I work on converting this into a book chapter.
Today, among other things, I've been reading a couple of papers, one a recent by Janniard on the steppe influence on Roman war making, the other, ostensibly, on Procopius' "new" Byzantine army by Breccia (2004). While reading these two papers (admittedly a touch more to go with Breccia) I've come across some interesting references to Huns I'd likely forgotten about.
There is, for instance, some extended discussion in Zosimus' New History on the Hunnic incursions in the last third of the fourth century (4.20ff). A cursory glance suggests to me that he either got some of his information directly from Ammianus Marcellinus, directly from Eunapius (fragmentary so hard to say), or even some sort of intermediary source. Much earlier in the text, while discussing Aurelian and his trouble with Palmyra, he also gets into some interesting - and ultimately successful - cavalry tactics. It turns out the Roman cavalry, when up against the Palmyrene heavy cavalry, employed the steppe-feigned flight. This interesting episode (1.50.3-4) raises three questions: is this just a stratagem employed by Aurelian for this specific context; is it a regular tactic employed by Roman cavalry in the third century; or is Zosimus projecting contemporary tactics (late 5th/early 6th century) on an event? I suspect we'll never know, though I'm sure someone or other has delved into this more deeply.
The Vegetius references to Huns are short and sweet: there are only two, one at 1.20.2, the other at 3.26.36. For all intents and purposes, what Vegetius says is that the Roman cavalry of his day is so good that the ancients have nothing useful to add that would be of use. He implies, too, in the first passage that this improved cavalry performance is attributable to the Goths, Alans, and Huns. In the second passage, where he repeats his belief that the cavalry of his day is sufficient in quality, Vegetius says that emperor (who's elusive, hence the dating issues - we don't know when it dates, late fourth century, first half of fifth century) is as good at archery as the Persians, and as good at horsemanship as the Alans and Huns. The implication of all this is that the Romans have adopted something of Hunnic cavalry tactics. Indeed, in his treatise on veterinary medicine (sadly I'm stating this secondhand), Vegetius praises Hunnic horses even more.
Before I turn to the dating question, much of what I've been reading about Rome's adoption (to whatever degree) of steppe cavalry tactics varies as to when and to what degree. The most recent case (save Breccia) is Janniard's, and he sees their big change as coming fairly quickly from the late fourth (first coming into contact with Huns c. 370 or later) into the fifth century. By the time they meet the Huns under Attila at the Catalaunian Plains, their tactics (and assorted things) have improved to such a degree that they're able to emerge from that battle victorious.
So let's now go back to the date of Vegetius' military treatise. For some, even most, it's the last quarter of the fourth century, and for even fewer it's some point in the second quarter (or so) of the fifth century. This new-found awareness of Rome's adoption of steppe tactics has me leaning even more heavily towards the fifth than I had been before. As you can see, these are extremely circumstantial pieces of evidence. We have two comments in Vegetius that imply they've already made big changes to Roman cavalry, and that they've adopted a Hunnic-approach. Given that the Huns first enter history for the Romans around 370 (some might differ on this date), and then peripherally, at least for a few years, it seems unlikely that the Romans would have made wholesale changes before really meeting them in battle. Ammianus implies that the Romans hadn't really come up against the Huns before Adrianople, despite what Zosimus might be implying (and there are all sorts of questions about the reliability of his text). If full interaction doesn't take place until the end of the 370s or even the 380s, though more likely the former, how likely is it that the Romans would have been able to make progress with their cavalry thanks to the Huns (Veg. 1.20.2 - nam licet, exemplo Gothorum et Alanorum Hunnorumque, equitum arma profecerint) between the late 370s and the death of Thedosius I, 395 (the latest possible date for the early-dating folk)? Guess that largely on how quickly they could change their cavalry. Given too that the Notitia Dignitatum (ND), which for the east is now thought to be spot on, the half of the empire most likely to need steppe tactics, and that we see little evidence for an overwhelming shift in that direction, I think we have to entertain the later date far more seriously than most scholars usually do. Indeed, most of the steppe-like units that we find in the eastern half of the ND are based in the east, not in the Balkans.
Yes, this is all mostly (entirely?) circumstantial evidence, but I think it makes the case for a later date for Vegetius' Epitoma Rei Militaris all that more likely.