Hey! It's probably easier than you think to get a vampiric sim.
The first thing you need is the Late Night expansion pack. Assuming you already have it, then complete the basics. You can place your family in any town, if not in Bridgeport vampires will be randomly generated from the sims of that neighborhood.
After creating your family, and selecting a lot to live on, find a vampire. One way, which I find the easiest, is to go to a "Vampire Lounge" lot at night, or in the day, though night is recommended as that is when vampires are most active. The other way is to get a job. There is a good chance that someone at your workplace is a vampire.
After finding a vampire, you must be at least friends before you can ask the Vampire to bite your sim. It depends with each vampire. After two or three days, well... welcome to the undead! PGR7-Habla 00:13, July 19, 2012 (UTC)
With Supernatural, you can make a vampire in CAS.
In Late Night, I went to the gym in Bridgeport, paused the game, and looked around. Vampires are easy to spot - their skin glows, and it is especially obvious in sunlight (and when compared to humans). Then, befriend the vampire, and ask him/her to turn your sim. The vampire will bite your sim, and in a few days, your sim will become a vampire. Enjoy! -Playeroflif3.