Couple of things, for one, Vecna is immune to Mind-Altering affects:
http://home.nitkin.net/dndbooks/DaD_DietiesAndDemigods.pdf
Page 94
"Divine Immunities: Ability damage, ability drain, acid, cold, death effects, disease, disintegration, electricity, energy drain, mind-affecting effects, paralysis, poison, sleep, stunning, transmutation, imprisonment, banishment."
Additionally, Vecna isn't a God of just 1 domain, which are his concepts, Vecna is the God of 3, meaning she'd have to somehow do this 3 times, that's implying that Vecna doesn't do something by then, before he loses his domains, but even then, he still would be a God and would passively get back those Domains as he has the Divine Ranks, which are what classifies Gods as what level Gods, for the Source I am using (Which his the Cannon 3.5E guidebook) he's only an Intermediate God, Greater Gods gain additional levels in their classes, around 4-10 additional levels and also have a Divine Rank of 20.
So yeah, Vecna isn't going to be affected by her Mind Manipulation, as the Lady of Pain couldn't do it in her Restricted form which by just speaking words, created the 3rd edition.