When did the Roman Army decline?

[ValentinianVictrix]

Hmm then why was Vegetius miliary writings so influential in the Middle ages and even the Modern era?

Vegetius was influential throughout the Middle Ages and through to the end of the eighteenth century because his Epitoma is chock-full of practical advice on recruiting, training and managing an army in the field that was relevant to armies of all ages, at least until the advent of modern mechanised warfare.

The Clarke translation of Vegetius was being used by the US military even up to 1946 as I have a copy of the US Military School publication of Vegetius from 1946 that was issued to officer candidates in the US army.