Lichlord Vecna appears to be a slightly more offensive Acererak, while they do share the same spells and slots, Vecna does have afterthought, flight of the damned, and rotten fate. Unlike epic Aecrerak however, he lacks the mythic trait which would give him more survivability in the short term. in the long term, Vecna can heal by using his Vile Teleport which heals him for 80 hit points every turn!I read the whole thing before he fixed it, and I didn't even notice. What IS the main difference between the two of them anyway? Anyone?
Lichlord Vecna also has three reactions one of which is an uncounterable counterspell. making him more defensive against spell casters. while Acereak gets his "Staff of the Forgotten One. He wears a talisman of the sphere and has a sphere of annihilation" Vecna has the book of darkness, which grants him all its positive traits at no downsides.