According to Vegetius, what led to the changes in Roman military armor and training techniques?

Weapons training was deemed of such importance that weapons instructors generally received double rations, whereas soldiers who didn't achieve adequate standards received inferior rations until they had proven in the presence of a high-ranking officer that they had attained the demanded standard. (inferior rations: Vegetius states that their wheat rations were substituted with barley)

After completing the initial training with the sword, the recruit was to master the use of the spear, the pilum. For this the wooden stakes were put to use again as targets. The pilum used for practice was, once again, twice the weight of the regular weapon.

Vegetius notes that weapons training was given such importance that in some places roofed riding schools and drill halls were built to allow for training to continue throughout the winter.