Actually rereading the adventure, it looks like Vecna does not know "abilities", just spells and such. So he wouldn't necessarily know Invulnar has a low wisdom save. Still, not uncommon for a caster to open up a wisdom save against a lot of fighter types to swing things around.The Dominate Monster play is a pretty nasty one from Vecna, though a spellcaster with a wisdom save as low as mine might be rare its not unheard of (and for fighter types it would be more common), but with Vecna's ability "see stats", it lets him always target someone how who will autofail their save. That's a big deal, as it means the advantage on the save for being in combat and the save every round are rendered moot. Seems a solid opening move for Vecna in the future, effectively turning a 4 v 1 into a 3 v 2.