Vatras |
Yes, all negative levels stack.
But when you look at the description of the vampire, you find:
Energy Drain (Su): A creature hit by a vampire's slam (or other natural weapon) gains two negative levels. This ability only triggers once per round, regardless of the number of attacks a vampire makes.
As DM I use that for all energy draining monsters since an incident in 2nd Edition. There was a three module series about mindflayers which introduced a vampiric mindflayer, whose tentacles could drain up to 8 levels per round.You can probably imagine what happened, when the party met two of them. Of course, back then existed no saves against energy drain and there were no negative levels, you lost levels completely. But the point stands :)