I know this is real late, but just posting in case anyone is curious. The last couple nights I did a couple Vulak runs and he only spawns after all the lord's and lady's are killed so the summon mechanic is redundant. Not sure about Aary helping the others as it was just easier to kill him first.. I...

He is definitely one of the hardest I've fought so far. I basically sacrificed my mage by pulling with the highest dispel to the opposite side of the room. You can get off 4-5 dispels before he reaches your group that way. It only takes 2 dispels, but he does resist quite a bit.