I guess I don't know what the book or his spells are doing at this point. Spells could certainly make it
worse, if he needed to cast any.
Near as I can tell, this is the ballgame for a party that can't create cover without casting. And, honestly, if the party were less caster-dependent, he probably just sits in his magically obscured pillbox and lets them eat not-spells while they try to figure out how to get at him without casting a spell.
Maybe I'm wrong.
Again, not bitching -- it's a cool lair. But...it's the lair. You had players who couldn't find any better tactical option than casting 9th-level spells and hoping Vecna gets unlucky, entirely because of the lair.