Instructions
Complete the "Story" mode of the game to unlock the Free Play mode. Once complete, click "Free Play" mode from the Main Menu. Enter the testing cheats mode code by pressing the "Ctrl," "Shift" and "C" keys at the same time. You can do this anytime during gameplay, and it does not mater if you have already created a Sim or started a family.
Enter "boolProp testingCheatsEnabled true" into the cheat screen to unlock the cheat testing mode. This cheat enables many features in the game, particularly new clothing and skin tone features in the Create a Sim screen.
Hold down the "Shift" key and click on the Sim you want to turn into a vampire. From the many options that appear, find "Make Vampire" and click it. Your Sim will now turn into a vampire.
Turn your Sim back to normal by repeating the previous steps. When you click on the option to make your Sim into a vampire, this time she will turn back into a normal Sim. You can also purchase the "Cure Potion" from the store at any time to bring your Sim back to normal.