Uni-the-Unicorn!
Hero
Vecna can cast dread just like anyone else casts counter. The only difference is that Vecna doesn’t have to see the act of spellcastingBecause the wording is "Vecna utters a dread word to interrupt a creature he can see that is casting a spell". That doesn't really work out if he counters after the spell goes off, unless he's breaking the laws of causality, which I guess would be cool, but I wouldn't call it the raw or how it's intended. That he needs to react and say a word means that he can't do it if he's unaware of the casting, imo.