Vampirism weapons improve in attack speed and damage by attacking mobs, but this does not happen with affixed ones. I suspect this will happen with other items with dynamic attributes as well.

This was a side effect of the way Item Attribute Modifiers used to work before the addition of the ItemModifierEvent. The upcoming update has moved affixes to use that event instead of NBT modifiers.

Just gonna piggyback off this and ask. I made a datapack that allowed bosses to have other sets of gear, but they don't appear to be showing up when I'm just spamming the boss spawner item. Does the current version read data packs?

Current version does, but the format has changed a little bit. Check the files in the published jar, rather than github, for examples.

Looking at the published jar, I do not see any boss sets that include bows, yet I observe bow bosses spawning in the world. Why is that? If this is due to my own sets loading, why am I not seeing the rest?

Yikes. I set the weight too low; never mind then.