When did the Roman Army decline?

[ValentinianVictrix]

Vegetius true or not, the is beyond a shout of a doubt the army by the time of Julian was no way the army of a century earlier or even the quality of Constantine's army. It is estimated the only 1/3 of the total Danube army was deployed at Adrianople. Army of Thrace, Army escorts I and II in the area would have 60,000 men on paper. Only 20,000 showed up at Adrianople. The condition was so bad that they had to recruit barbrarians by the time of Theodosis.

My advice is to do some more reading on this matter.

Most Late Roman armies numbered approximately 25,000 men strong. That was considered more than enough to carry out the task needed. Only invasions of Sasanid Persia required larger sized armies, Julians was estimated at about 60,000 strong. But even then he divided his force into two parts, sending one North into Armenia to then move south-east to catch the Sasanids in a pincer movement which never took place.

Valens was recruiting from the Goths from before 376AD for his proposed Sasanid Empire invasion and some estimates put the force he was going to use at 90,000 strong. The Romans had recruited 'barbarians' for centuries, so I don't understand why you believe this was indicative of the Roman armies decline.

Valens probably did only take the standard size army, 25,000, with him to Adrianople because he left over double that in the Eastern provinces to guard against Sasanid incursions, which had been happening from 368AD onwards. He also did not need to strip the Eastern provinces of troops for his campaign against Fritigern's goths because his nephew Gratian, Emperor of the West, was moving east with probably a similar sized army, both armies had they combined would no doubt have completely wiped out the Goths, leaving them little more than a historical footnote.