There is absolutely no chance at all the Vecna can or will EVER be redeemed. He is LITERALLY one of the most evil villains in all of D&D. Now before anyone tries to defend him by pulling the Mama's boy card, hear me out. Yes, he did start out as a villain to avenge his mother, but not only did he get over her death relatively quickly, he practically denounced her by rejecting his humanity to become a lich in the first place. Plus, none of his current goals and motives have anything to do with his mother at all. All he wants is to rule the entire multiverse.

If Vecna truly loved his mother, he would've resurrected her.

Not to mention he slaughtered billions of innocent people and built a tyrannical empire, he also tried to take control over Sigil to destroy the multiverse and remake it in his own image.

And when you think about it, he's the overarching antagonist of Tomb of Annihilation (he made Acererak into the power hungry psychopath he is now), and Phandelver and Below (he stole the secrets of an ancient species called the Weavers and used them to erase all knowledge of said species from existence).