Orcus is a CR26 monster though, he was my point of comparison. He has potent offensive spells and the ability to call up powerful undead like liches. Vecna can be defeated by a potion of Necrotic Resistance.Because that is the power level of a CR 26 monster in 5e. I think people have preconceived notions of the power level of particular CRs without investigating what they really are![]()
But you are definitely a better judge of CR than I am, Dave

Vecna n Miska: