As stated in a Twitter session with Grant Rodiek, vampire Sims are also able to grow plasma fruits, which they can buy from the grocery store, at night in their own personal gardens. Vampires will also be able to eat plasma packs, which function the same as snacks for human Sims. A vampire can also raid the hospital for Plasma packs, or raid the grocery store for Plasma fruits.
If Vampire Sims have the Vegetarian trait, they are limited to only plasma fruit, instead of plasma from other Sims. If a regular Sim ingests any plasma product, they will become ill. Plasma packs are only available to Vampires, and regular Sims can't drink from Plasma packs.