A couple of points. The story about the eponymous ancestor "Vyatko" seems a later construction. The East Slavic modification of "in-" and "en-" to "ya" is what suggest a Venetic name for them. "They came from the west": what is also interesting is that the material culture of the Viatichi is practically if not entirely similar to that of the eastern Galindians. So there's a lot of room for speculation here.