Vampirism didn't work against the explosive trents, but it worked against the overlord. Still, a single hit wouldn't be enough to heal his wounds when many explosions chipped away at his health. While the skill was leveling up and becoming more efficient, it still would take a while for it to be enough against that enemy. Time wasn't something that Luke had, though. Since he could sense the fire, Golems were fast approaching. Not to mention, he wanted to find Aivern and the others as soon as possible...