I think this is happening because of the many vamps running around in each town. That looks like a DISEASE!! Vamps are cool, but it has loosed the "thrill" when everyone is a vamp, because is extremely easy to turn into one. It sucks and is not blood!!
For those who play LEGACY... Myself included: We could "kick" the non-heir sims, with his/her housband or wife, along with 2-3 kids. They will find a house and begin having more kids on their own. Of course, there will be a lot of "cousins", but that will help a little. And for mating, your heir could always travel and look for his or her "other half" in France, Egypt or China.