Others could also have run stat blocks (the way they used to be) in any fashion they wanted, as well, but WotC felt such changes were necessary.
So, instead of people converting the old stat blocks to get they want, now I (and others) have to convert the new format???
Vecna has the spellcasting trait, so should abide by those rules. A game that insists on different rules/systems of monsters and another for PCs is not well designed IMO. Vecna could have kept his spells as was formerly done, and given him other features if DMs desired.
And creates
OTHER complaints in the meanwhile.