Game Help:Sims 3 Occults/Vampires
From SimsWiki
How to become a vampire: You need to get your Sim to meet a vampire and use the "Ask to Turn" social. Vampires are marked with a red border in your Sim's relationship panel.
Vampires do NOT die if they're in sunlight for a short period of time. If they stay out too long, they get a bad moodlet (Scorched), and when the countdown is over the vampire Sim will die.
The vampire Sim has a "Hunt" option if you click on the floor of a room with Sims in it. They can also raid the Hospital for plasma or drink Plasma Juice from the fridge.
Vampires can die of old age, though they live significantly longer than normal Sims.