i.e., with a POD after the Norse stop visiting the Americas, make their next visit from the Old World (that leads to anything beyond sailors' tales) be a Russian expedition. It can be from either direction.
All we need is a roving band of Russian Explorers replacing the role of Columbus. I'm sure such a character can be created--some kind of Russian noble who has had a feud with his family and now is a professional explorer. Not a bad story, actually.