Although he is just about the highest level spellcaster one will see in SLASH'EM (only Dispater, Orcus, and the dreaded Demogorgon have a higher base level), Vecna is nowhere near as tough as one might think. He is somewhat slow, and has only one spellcasting attack. His main weakness, though, comes from the design of his level; it has no up staircase at all. Being covetous, Vecna will teleport to meet you. Since there is no up staircase, he will not warp away like most covetous monsters, but will continue to teleport to meet you, even as his hit points drop. Elbereth, which he respects, is a must when fighting him, since his high base level means that his psi bolt spells will do immense amounts of damage. Additionally, one does need a +4 weapon to damage him at all. However, with a permanent Elbereth and a +4 weapon, he is quite easy to defeat, in many ways less trouble than the demiliches that surround him who will try to run away when you hit them. Also be aware that he will not attack until you come within sight of him, so it is very possible to choose when you fight him.
The Chaotic Quest itself can be a difficult branch, though. It is advisable to bring an escape item or two, just in case Vecna or one of the spellcasters around him summons a minotaur right next to you.