As a DM, I really do not like the "ready an action to release it so I can't be countered" tactic. It feels against the intentions of the game, and grants the PC's another advantage Vecna doesn't have because he'd lose out on attacking with Afterthought.
On the other....it does feel pretty cheesy, I don't like the "mouthfeel" of it.
Although it is a "cheap move" against Vecna, he was able to use it himself. And for him, with 3 reactions, he still had 2 remaining for other things later in the round.
Counterspell is an issue IMO because it is automatic against equal or lower level spells. It should require a check, regardless of level.
Anyway, until it gets changed, it is the only method of defeating counterspell, and has been noted it eats up your concentration, and if the trigger never occurs you lose the spell anyway. This technique also stops the counterspell-counterspell annoyance.
Woops! (Didn't matter in the end anyway).
Actually, if it applied to all the damage, Broodhuz would have survived the turn anyway--but the outcome was pretty much inevitable at that point.
The domination in our case proved to be exceptionally strong, but I think that was a particular weakness of our group (well me at least). Even with a +2 to the save I would least have a chance to break out (and now with advantage on the save + saves when I'm damaged) there is a chance to get out.
The barbarian was pretty much in the same boat as your sorcerer. Either of you could have been dominated automatically, but since you were flying (if you wanted) and could cast
chill touch, Venca had to stop
THAT from happening again.
Also parties with big single strikes might have been able to break Vecna's concentration.
Well, they force him to use Legendary Resistance anyway. (Which certainly is important in its own way, of course!)
I do like the idea of giving him shield, even if its just 1/day. I think that rounds him out a bit, prevents an alpha strike, gives him just a little chance to get his mojo going (which is pretty strong).
His AC is pathetic IMO. If a caster in the party can get a 20+ AC routinely, and Vecna
can't, that is a major liability!
Giving him
shield at least gives him the option, at would cost him a valuable reaction!
Otherwise, I think he's just too reliant on his teleport for durability.
Which is too limited IMO. 30 feet is pretty weak and PCs were able to still follow him enough. I felt like I was using his Fell Rebuke all the time.
Looking back at my battle analysis, in 4 rounds he used 10 Fell Rebukes and 2 Dread Counterspells, and he used his reaction once to release the Dominate Monster. So, a lot of Fell Rebukes...
Remember Frightened also prevents them from moving closer to him.
Yeah, I know, but at this level it isn't as important IMO, but might be more useful in the proper scenario...
On the Book of Vile Darkness it's also stated to contain any spells the DM wants, so it can be used to give Vecna some more spells.
But this goes back to an issue with the statblock---
he has no spell slots to cast spells with!
So, how would that work, now? It was a reason why I suggested substituting some of the spells he has, which also removes player knowledge a bit from the situation.