That all depends on where you're standing.The severing of the hand and eye of Vecna comes later in his life. It wouldnt be part of an origin story.
Typically, if Vecna makes an appearance as a villain, it's hundreds or thousands of years after his downfall. An origin story in such an adventure would certainly conclude with Kas's betrayal and the severing of the Hand and Eye; that explains how Vecna came to be... whatever the writer has decided he is at this point (god / demigod / god wannabe / spirit possessing the wielder of his artifacts / reconstituted lich / demilich / ???).
Now, if you were going to have a story set in the past when Vecna's empire was at its height, and Kas's betrayal had yet to take place, you would obviously push the origin story farther back. It would probably conclude with his transition to lichdom.