How does EVE handle this stuff? I have the impression they don't really have the traditional AI enemies; instead, the other players are the enemies. Do I have it right?
Some of each. It's been a long time since I played, but there are AI enemies. They're ships in space though, so they tend to explode on death; not much room for getting back up. They respawn, but very slowly. You'll generally only see pirate-type AI enemies in areas that are relatively abandoned by human players. The game world is maybe best imagined as a "network of bubbles", individual regions of space linked by warps between them, which may or may not be under the control of various player factions. So there aren't really spots like you see with most MMO's, with a sort of "field" perpetually filled with some respawning enemy type.
I'm curious about the world size. Vampires aren't exactly supposed to be outnumbering humans 10 to 1, so they'll need quite a lot of human NPC's wandering around, lest they end up with the odd situation of every single resident of an entire city secretly being a vampire, except for the one unlucky hobo they all feed off of. Assuming that the Masquerade is a thing to care about, combat would probably stay off the streets unless initiated by other players, leaving most AI enemies to be found in quest/instanced areas. Hard to say anything for sure, though, since to really pull of a WoD MMO they'll already need to be doing things differently than most MMO's.