To retrieve the eye or hand of Vecna, you must wait until a survivor collects one of them (indicated by a voice line along the lines of "that isn't yours" "you don't know what you took" etc... and a little icon next to their survivor profile on the bottom left of the screen).
Then you must get that survivor to death hook (the same way you would if you were looking to Mori them) and you should have an option upon downing them to retrieve your body part. This will act as a very quick mori.
From here all you need to do is kill the rest of the team!