He's a CR 26 monster. He's supposed to lay down a ton of hurt. Rotten Fate is pretty reasonable for his CR--in fact, it's on the lowish side.A little OT - Unless I'm missing something in the stat block, Vecna can cast his "Rotten Fate" ability without limits. IMO, that is OP.
Compare Vecna's raw damage output to what Zariel (also CR 26) can dish out, and Vecna comes out distinctly behind. He makes up for it with his defensive reactions, his self-heal, and his ability to shut off PC healing. Zariel hits you like a freight train; Vecna grinds you steadily into dust.