Vitae, in vampire terminology, is blood. It is what sustains the undead body of the vampire and the reason they must drink the blood of the living.
Characteristics
The primal essence and energy of the Cainites, the fuel of a vampire's Disciplines, their means of Embracing and also the cause of their cravings.
When the vampiric blood is consumed repeatedly by an individual, it has de property to create a supernatural link of fidelity and dependency towards the vampire who gave his blood, thus forming something called a Blood Bond
Other Theories
It is theorized by mages that Vitae is actually fused with Quintessence, which explains its supernatural properties.