First, you’re going to want to make a sim get their vampire lore up. This is a new skill introduced in the game pack and is increased by first “researching information on vampires” on a computer. Once you’ve found out everything you know there, you’ll need to progress through reading tomes that you order online. Continue this until you make it to the ultimate one, and level 15 in the lore skill will teach that sim how to cure vampirism.
Next, you’ll need 10 wolfsbane, 10 garlic, and 10 plasma fruit (all of which can be purchased from the computer after getting to level 15) before making the Ultimate Vampire Cure at any bar that will allow you to mix drinks. Simply drink it up and they will be back to a life of mortality.
http://twinfinite.net/2017/01/sims-4-vampires-how-cure-vampirism/