Hasn't Acererak become a vestige? 3.5 Ed Tome of Magic pag 19. And I warn the very-smart layout of this book had to be imitated in the 5ed style.
My crazy intution says Vecna hasn't said his last word about the dark powers and the demiplane of the dread (Ravenloft), even Vecna would dare to create his own "demiplane of dread" to take revenge against previous enemies, or some domains as "buckups" in the space-time continium to avoid time-paradoxes caused by chronomancers and company, or to keep prisons who are reincarnated into new lives forgetting totally their past, because they are very dangerous, but also potentially useful as a joker-card against other enemy in the future. This "demiplane of the torments" would be within the plane of the mirrors, or only can be found by means in a secret planar-portal within the plane of the mirrors. One of these torment domains would be a grimmer rip-off of Fortnite: Battle Royal. Other torment domain would be almost a time-loop. Everything is normal until a zombie-apocalypse starts, but the fallen victims returns to the past, with all the memories, maybe stronger thanks the experience, but usually suffering post-traume stress. Most of the times they are await this zombie outbreak, but this doesn't happen.... until any week laters, or a couple of months. Then they are enough ready, and even they don't fear the death, but the suffering causes injuries in the soul.
If Vecna is now one of the most known D&D villains thanks "Stranger Things" I guess Hasbro will want more "product emplacement" in the Netflix series.
I wonder if 5 Ed is ready for a "for mature audence" sourcebook as a "Vecna's Book of Vile Darkness" or a "Azalin's Libris Mortis". Maybe is +16 instead only +18 to avoid censure in other countries.
The 5Ed version of Vecna is the best design, but I would add an crystal-eye and a prothestic gaulent to give a more dreadful image.
What about Kas the vampire? The last one I read about him this has become a vestige in Dragon Magazine. If vampires enjoy regeneration then Vecna could creat lots of Kas' eyes and hands to take revenga against his betray.