Have your Sim become friends with a Grand Vampire. Make sure they have a high Logic. Later on, your Sim will get bitten, turning him or her into a vampire.

Enable the boolProp testingCheatsEnabled true code, then click on your Sim and select "Spawn". Then, choose "Sim Modder" and go to "More". You can now select "Make Vampire". Note: If you out in the morning you will start to quickly melt -- stay in your coffin.

Become friends with a vampire, then have another resident of the house influence the vampire to bite you. If your friendship meter is high enough, your Sim will allow the vampire to bite.