Well, here's an update!
In my game there were 2 births today, and both households are mixed human+vamp. In one house there was a twin birth (!) and I rushed in to age up and check if there were any human babies. Uh, nope. 3 more vamps are born. No humans.
I now wonder if the game is programmed to balance this, you know the way boys/girls is balanced. Or is it balanced because it's 50/50? I already have 300+ humans in my game, so I really hope there is no such balancing thing - like giving priority to vampire births until they are 10% of the population...? Ha, probably not a good idea.
I'll need to give birth restrictions to my little vampire community. From now on only 1 offspring per vampire or thy will be tied to a fence in the morning on a sunny day... muha!