Von Leon

Best I can assume is that either the notices are time-based, and show up when you lower it's HP a certain bit, or that the act of summoning mobs in itself counts as an "attack" and therefore he's stuck in limbo for that too.

This is what I'd assume too - mob summons are based on some constant, and VL's stuck in some [seemingly] infinite loop on trying to attack and failing. The way it's being described it's almost like if it's being hit fast enough it's resetting him from "aggro'd and casting attack" to "just started aggro determining attack" or something. I don't know how they have it set up to determine how a boss chooses attacks, but that's the best guess I can come up with, that it's being knocked through the loops set up for that and can't get out unless players go slow enough to let it.
./ramble