Vecna's reactions should allow saving throws. Just for gameplay fun reasons.
Strength on the "push and knock prone" AOE, and Concentration saving throw on the counterspell.
I would probably also add an ability for Vecna to pick someone to gaze at. A creature gazed at has disadvantage on all saves against Vecna's spells and abilities, and Vecna can see them no matter where they are. Changing who Vecna gazes at is a Divine Action.
As it stands, you end up with a funny puzzle where you first block line of sight before casting spells, or you avoid being a melee PC entirely and use ranged weapons, or get almost all spellcasting nullified and half or more of all melee attacks nullified with no recourse.
Divine Spellcaster overlaps with Spellcasting feature. It states you can cast every spell as a 9th level spellcaster for 6 divine actions; then you say he can cast Wish for 6 divine actions. Can he cast wish using both features for 6 divine actions, twice?
Myself, I might give Vecna a spellbook. It has every spell in it. Make Vecna turn the pages to find a spell, give it HP so it can be damaged, etc. Then the "every spell in existence" consists of Vecna turning the pages of the book to find it (maybe using up a divine action), then casting it (on a different turn). Again, setup and deploy situation, maximize fear.
Ah, the "turn the page" action could involve him naming the spell in Abyssal? That results in some wonderful "he is about to cast what? Oh naughty word".
The spellbook might be nearly indestructible, but if it takes 100 damage the page Vecna has open gets smudged and he can't use that page until the next rest.
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Where are Vecna's actions? I guess you intend for him to use the same pool for legendary and non-legendary actions. I don't like the pacing implication - the point of legendary actions was to keep the threat of the solo monster spread over more than a single point in the initiative count.
Tactically, using your oomph as early as possible is almost always the right answer, but it gives worse dramatic gameplay and makes it less interesting for PCs. So we end up with a tension between optimal tactical play of Vecna and optimal dramatic play, which sucks.
I'd return to the on-turn action/off-turn action model somehow. And strongly resist the super-expensive super-powerful actions, or give them a recharge; recharge mechanics on off-turn actions are great, because you can drop the fact they have recharged on his turn, giving the players 2 turns of "oh crap" preparation time.
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You should work in the Eye and Hand of Vecna into his statblock. An idea for the Eye is a Divine Acion:
Eye of Vecna: Vecna puts his gaze on a creature who just finished their turn. You can feel his eye in your mind. This gaze remains until Vecna uses this ability again. Vecna and the Gazed upon creature always have each other in line of sight. The gazed upon creature has disadvantage on saves against Vecna's abilities and Vecna has advantage on attacks on that creature and ability checks on that creature.
The idea is that Vecna picks out who they most find a threat, and Gazes at them. The eye pierces all shadows.
But I guess this overlaps with Extract Secrets. You could just rename it to Eye of Vecna as an alternative.
Note I included a slight downside - when the Eye is on you, you can see the Eye, and it counts as line-of-effect both ways! The range to Vecna remains unchanged.
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For the Hand of Vecna, I might have a spectral hand that is a cross between mage hand, bigby's hand, and spiritual weapon? That grabs and restrains a creature Vecna chooses, draining their life and blocking healing?
Hand of Vecna: A ghostly hand grabs at you. Melee Spell Attack, +14 to hit, unlimited reach. The creatre takes 3d12 cold (19) and 3d10 necrotic (16) damage, suffers 1 level of Exhaustion, and is Grappled (escape DC 26), Restrained, and unable to regain HP. The creature is released if Vecna uses this ability on another target. At the start of the creature's turn while grabbed by the Hand of Vecna, it takes another 3d12 cold (19) and 3d10 necrotic (16) damage.
The Hand can be attacked; it uses Vecna's AC and Saves, and if it suffers 100 damage it releases its target. (this is not a super-efficient way to break free, but why not).
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This replaces Draining Touch. The Exhaustion only triggers on the initial attack, after that it just holds onto you and sucks your life out slowly.
In addition, this gives Vecna a reason to use Afterthought; you already have a target pinned with the Hand of Vecna.
