So in theory, vamping, say Barty Crouch Jr would almost bring him back to life, wouldn't it?

I think the demon would take up residence, but it wouldn't have access to Barty's memories and personality. So it would pretty much be an amnesiac vamp. On the other hand, Fred's soul was destroyed and Illyria still had access to her memories, so I could be wrong. (I've always felt that the difference between Illyria and vampires is that Illyria had this long life before she took over Fred's body, while the vampires are newborn and mold themselves on the only personality they know. But I'm not sure that's a correct interpretation.)