This can be true but that's one of the reasons I just avoid placing someone like Vecna in a position to be targeted at all. He is a force that should be untangible in regards to the party. If you stat it the potential for your party to destroy something that supposedly has thousands of years experience and knowledge will just deflate the game. That's why I said I'd use shadows instead. You might occasionally beat a sliver of his actual power or temporary slow one of his plots. Are you going kill him? No. At best you will set in motion one of many contingencies plans.