I had always presumed that while vampires as a mythology are technically undead, that as creatures they are still "human" enough that the living rules just applied to them moreso than the non-living ones.
To me the non-living as a category more speaks to things like skeletons, ghosts, golems, and things that dont operate under the same rules and physics of life. So while a vampire is an undead creature, its still a flesh and bone being that is capable of receiving healing and has a working, aware mind and personality. Thus the living category "fits" it better mechanically.
That was my line of reasoning for it anyway.
