If you can get the Mityvac to pick up from the bottom of the diff, I think it should work but it could struggle with the thicker gear oil. I would try it with the diff oil warm.
Pulling the diff cover isn't a bad idea if not messy. I would go that way to avoid any drama. If you were planning on swapping diff covers to a Mag-Hytec or similar with a drain plug it's also a good time.