Ref the UOP changelog:
Changed the Human Blood "potions" found in Vicente Valtieri's quarters and the Pale Lady's lair so that if the player is a vampire, drinking one counts as feeding (Vicente and the Pale Lady can feed from them, so the player should also be able to)
The logic behind this doesn't actually work, it interferes with the core element of the vampire game-play loop. The issue came up in discussion whilst working on UORP but then wasn't back-ported to the UOP.
Vicente Valtieri and the Pale Lady both display advanced stages of Vampirism, despite having access to bottle human blood. Unlike the player, NPC's (theoretically) need to eat, and the bottles of human blood appear to be intended to represent "regular" food for the vampires. There's no evidence it was meant by the developers to replace "live" feeding, which resets vampirism to the first stage, unlike drinking bottled blood.