Late Roman Army during the 5th century

[Flavivs Aetivs]

I would argue that the Notitia Dignitatum was an Ideal organization of the Army listing paper strength units; the army itself probably only numbered around 90,000 in the West, and about 12000 in the East (Paper Strength). This is supported by the fact that the Western Empire had to cut off pay to ~~30000 troops after 439, and probably only had around 30000 left afterwards (about 5000 in Italy based on some later figures under Ricimerus, enough in Illyria to conquer Italy absed on Marcellinus' claims, and about 12000 in Gaul with Aetius consisting of Private Retainers [Bucellarii], remnants of the Field armies and Palatina units, and pseudocomitatenses [re-purposed Limitanei])

The west fell because of money - the west never had the money to sustain an adequate defense force without help from the Eastern Empire's rich provinces of Egypt and Anatolia.

As for the mention of Auxilia, have you considered Vegetius is just referencing the Limitanei using a different word? It was easier in the Limitanei and they often settled down in the area they were stationed (a problem in that day for the army when mobility was considered important) in the communities that sprung up outside the forts.