When did the Roman Army decline?

[Renatus]

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.