I don't know, Vampires between Weak Skin, but immortal... I admit it does seem weird that they can pick up human diseases... but maybe they should have their own diseases?
I'm Just saying @SimGuruNick...
If they bit someone, drank blood/plasma from a sim who was sick perhaps it should do something to them.
In my belief Vampire Lore, I think it is possible for a vampire to be ill, Just because you are immortal doesn't mean you have a super immune system... I'm just saying instead of patching it as A Fix, maybe add it as a additional Strength or Weakness