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). 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.
Blood packs are commonly found in first aid kits and hospital settings across the wasteland and can often be purchased from merchants at little cost.
Behind the scenes
The quest is named after the popular phrase uttered by Bram Stroker's Dracula.