Vanilla can also refer to the most basic unit of a type regardless of mods. For example, archers are "vanilla" archers compared to turcoman foot, trebizonds, etc. Spearmen compared to Feudal Sergeants, Saracen Infantry, etc. and so on.

Reinforcements are supposed to come in from top to bottom, but every once in a while it doesn't work for me. I'll have all my guys carefully arranged in the order I expect to need them, then when I start calling for help the order is completely different than what I had set up

Usually it works fine, though.