I think when a person becomes a Vampire (in some movies) they are possessed and the spirit (or whatever) that now inhabits them uses their memories to mess with the people who knew them like Richie did with Seth.
But as said before this is not an intellectual movie exploring the mythos and mind of the Vampire. If you want that Shit watch Interview with a gay Vampire. But this is a popcorn flick so don’t question the green blood, just except it and love (or hate) it. 
