Legionary Officers and NCOs - Late Roman Army (284 - 565 AD)

[Robert Vermaat]

So if Vegetius did see unarmored infantry, that was nothing new.

I agree. Plus, the lines where Vegetius describes the lack of armour are not from an observation, but he claims to describe a formar process in which the soldiers appealed to the emperor to get permission not to wear armour:

Vegetius (1.20.3): But negligence and sloth having by degrees introduced a total relaxation of discipline, the soldiers began to think their armor too heavy, as they seldom put it on. They first requested leave from the Emperor to lay aside the cuirass and afterwards the helmet. In consequence of this, our troops in their engagements with the Goths were often overwhelmed with their showers of arrows. Nor was the necessity of obliging the infantry to resume their cuirasses and helmets discovered, notwithstanding such repeated defeats, which brought on the destruction of so many great cities.

Especially the second part sound like fantasy to me.