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Stroll down to the Santa Monica pier and speak to the guy on the beach in the red rabbit t-shirt. Having watched way too many vampire movies, he seems to think that if he tracks down the "head vampire" and kills him, that all other vampires will become human again (movie: The Lost Boys). With a bit of Persuasion, you can convince the Thin Blood that you can only kill the "head vampire" with a Holy Stake. He's interested in purchasing such an artifact from you, but he also wants to know if a full blood transfusion will cure him of his vampirism as well (movie: Near Dark). Again, with a bit of Persuasion, you can convince him that this is possible only with unicorn blood.
Walk down the beach to the chain link fence, where you'll find a Wooden Stake
just at the edge of the water. Grab it, then check your inventory to make sure you have some Blood Packs
(yep, just normal ones). Once you have both items, return to the Thin Blood and strike up another conversation. With some Intimidation, you can convince the guy to pay you the money for the items ahead of time, which will earn you $200. Otherwise, you can actually sell him the "Holy Stake" for $100 and up to three packs of blood at $120 a piece. As soon as he has the stake, though, he runs off into the night to track down the notorious "head vampire".
Note: This encounter will not appear in your quest log.