I Want to Drink Your Blood

 refers to an unmarked and repeatable quest in Fallout 3.

Walkthrough
After finishing Blood Ties, Vance will happily pay you 15 caps for every Blood Pack you bring him, even though you could have mentioned "donating" them in your conversation (you also had the choice to have Arefu sell them). No experience or Karma bonus is granted. In order to 'sell' Blood Packs to Vance you simply need to have them in your inventory when talking to him after the main quest is over. An additional option to 'donate' them should come up. Choose this and Vance will give you the appropriate value in caps in return.

The Vendor downstairs will sometimes sell bloodpacks - often for less than 15 caps.