If he's so capable of avoiding simplistic maneuvers, let's see him clap.
Oh. Right.
As D&D's most cartoon supervillain (more than the actual cartoon's supervillain), Vecna might be smart, but he ain't bright.
Forgetting that this is the "whole" version of Vecna,
You'd think, with his secret knowledge, Milenia experience and all answering "what is the sound of one hand clapping?" would be a cinch for the guy.