I really wished you could make a “squishy” boss like the published Vecna work without giving him spells that are obviously good on anything. I would rather give him some more HP and the Book of Vile Darkness (as a DM you can get up to +3 ac out of it, if you want).
I very much dislike the shield spell, and that it is the best spell in the game because it’s relevant from level 1 to level 20 in any game that involves getting hit with attack rolls. It is so centralizing to the game that every character picks it up, or even goes to great lengths to acquire it through multiclassing. I rather ban or nerf the spell that giving ALL monsters the shield spell because it is the only way to have spellcasters make sense.
For Vecna specifically, he already has a reaction that should work against martials, the teleport. Sadly the original adventure just tosses Vecna in a white room to get blasted down. Isn’t it more interesting to have Vecna do something different?