Removes any resistance. You can contract it from melee strikes, too, the odds of contracting it are just increased when they use Absorb Health. I tried the method of having them solely use Absorb Health, and it took a loooot longer than just standing there and occasionally popping a healing potion as needed.
No kidding? I thought that was just in Oblivion.
Anyway ..yes the HS cave is the best place to go. I enter, kill the first Thrall and drag their body to the cave entrance. The vamp will raise them if you don't. Then go down the tunnel and get the attention of the first Vamp,
Run back to the room where you killed the Thrall. Run up the ramp and wait for the Vamp to come up there. She will start with the VD from range. It works on a per shot basis, so if it doesn't get you at the start of the cast, don't stand around. Jump down to the ground and wait for her to come down. Run around her and get back up the ramp. Wait and jump off again. Keep doing this until you get it. This cycle will keep breaking the spell and you do not have to chug a hundred potions.