Blood

Blood is a red, life-sustaining liquid that flows through most creatures.

Signifiance
Blood is the currency of the soul and the very essence of a creature. To drain someone of their blood is to absorb their soul out of them.

To Vampires
Vampires require blood to live, or else they will grow weaker and weaker, then slip into a state of near death. As well as being sustenance, draining blood from a being -- be they monster, human, or animal -- allows the vampire to store them inside their body as a familiar of sorts. The souls inside of a vampire may also contribute to their immortality, with each "death" being one soul spent. This would explain why Alucard is seemingly perfectly immortal, as he has drained entire armies (animals included)-and-then-some worth of souls.