Vegetius\' description of a Roman Legion

[Sean Manning]

How much evidence do we have (more evidence -> a higher level of credence is possible if the evidence mostly agrees)
What does other evidence say?
Why did our source say this? Were they an eyewitness? Talked to an eyewitness? Read it- where? Hearsay? Didn't believe it at all but was trying for rhetorical effect?

Most of these questions are impossible to be answered for any source in Antiquity.

Isn't that overstating it? In the case of Vegetius, for example, we are pretty sure when and in what context he was composing; we know some of his sources, although they are all lost; and his general method and purpose (he is digging in Latin books to learn the good old way Romans used to fight in the belief that this will solve the military problems he perceives in his own day). We are rarely certain of all of these things, but the same goes for most historical sources.