When did the Roman Army decline?

[Frank]

Vegetius doesn't seem to be an unreliable source, he actually knows how military training and organization works and he is still studied in modern military academies.

Vegetius was probably an armchair general, who condensed about 4 ancient sources about the imperial army. In doing so, he mixed up eras greatly and created a Neverland-Legion, which never existed. According to Speidel and other historians, his weird figures are not worth to discuss. Most likely he just made the usual rookie-mistakes while calculating legion strength, I did too, when I was 12 year old. He simply found no exact figures in his sources and started speculating wrongly.

Of course some chapters are worthful and more correct than others. But his intention was not accuracy but to get a message across. Perhaps he never saw a roman legionair.

However, I can imagine, that training and discipline in the late roman army was partially worse than in the early empire.