My take on Vecna is absurdly overpowered, so fair warning.
I'd have spell-like action options, like in the new monster stat blocks, while also keeping a large number of utility and divination spells available. His go-to, at will, cantrip-equivalent power ought to target multiple enemies and do buttloads of damage.
He would have lots of hps, but not ridiculous amounts- but lots of ways to self-heal (generally by absorbing life from enemies), soak or avoid damage, and end conditions and effects on him. He'd counterspell like crazy.
He would be very tricky. His wouldn't be a stat block for beginners. You'd need to spend some time with it to really use him right.
I'd probably also take something from 4e and give him some action points that he can use to take an extra action on his turn.
Finally, and here's the one that makes him truly, irreparably broken, he would roll a d30 instead of a d20 on attacks, checks, and saves. This was one of the main features he had back in my 2e campaign, and it makes him a terrifyingly competent foe.