Legends Viewer would still show the vampire, but it shows their origional name I think, not their assumed name.
There are ways to spot a vampire without diving into legends anyhow.
Thanks. I wasn't really clear in my post, but I wasn't planning to use it for vampire hunting. Just to have some sense of the world my fort was in, and to make sure I don't align with a dead Dwarf civ or something. My question was more hypothetical: if a migrant is NOT in Legends, then does that mean he/she is an assumed name?
So if all migrants are historical figures then my suggestion would still be for vampires to assume pre-existing identities, that way you would see a history for the migrant. For extra awesomeness, the vampire can attack someone who really was planning to migrate then assume that identity, but that's probably several versions in the future.
"Hey, Urist McImposter, there are tales of you slaying a Giant Cave Spider with your sock when you were just a lass. And yet for some reason you don't seem to know the first thing about fighting. Weird, huh? But don't worry... I'll still vote for you for mayor."