I have had Sims who were born hybrid witch vampires (twins). It didn't work correctly though. It was years ago, so I don't remember all the things wrong, but I know they had different needs, perk trees, etc. that didn't go together and there wasn't a magic meter. I was afraid that it would somehow mess up my game and ended up curing one of vampirism so she could be a spellcaster and the other of being a spellcaster so she could be a vampire. On the plus side, any child of a spellcaster, vampire or otherwise, still carries and passes on any spellcaster blood traits.