You could try having a live in human Sim for your Vamp to snack on, if you don't mind doing that. I would suggest going out to festivals, bars and other community lots to find the non Vamps. You can try forcing the game to spawn them by hiring Maids, Nannies, Butlers and other Npcs, I don't know if they will be Vampires or not but it's worth a try, if your Sims can afford that is, you only need to hire them for one day to meet them and find out.
I recommend reading the Vampire guide here too.