"Let's see, vampires have the ability to recover from most injuries they receive.
With ordinary outside force...... I mean normal weapons. With such things, you cannot cause wounds greater than the speed of their recovery.
To kill a vampire, you need an outside force greater than their speed of recovery, or something that nullifies their ability to recover.
This ability of theirs, the curse of restoration---actually isn't treating the wounds. Instead, it is a reversing of time to restore the damaged body part back to its original state. An occult artifact which nullifies this effect is called a conceptual weapon."