If any vampire hunter is ever in a position to get essence drained, you have far worse problems than magic loss. For Essence Drain to work, the target has to either be totally willing or totally unable to resist. A character that is being Essence Drained is either a very close friend of the vampire, a vamp groupie, an essence drain addict, or totally bound and (in the case of a magician) blinded. Heavy sedation may or may not work because of the strong emotional connection requires; it depends on the drugs used and the circumstances.

And yes, I did say Essence Drain addict. Being Essence Drained is both extremely pleasurable and extremely addictive. A vampire hunter who is captured, drained, and somehow escapes will have, at the very least, a mild addiction to Essence Drain.