I don't know how you could design a stat block that can keep back entire organizations of 20th level characters lolFar more spells, especially useful ones. Spell slots so some can be upcast. That means 8th and 9th level spell slots.
Cantrips, like Prestidigitation...unless Venca plans on opening a dry cleaning service, why is this even on the list?
Venca of course adds his prof bonus to ALL saves. Utterly ridiculous that a CR 26 has a +2 to Str saves, or a +2 to ANY saves.
Vecna should be something an entire conclave of 20th level clerics, paladins, and wizards should have to work together to cleanse the world of for a period of time.