How easy is it to contrive a set of circumstances where the great invasions, migrations and raids of the Vikings do not occur?
What if the male population of Scandinavia were continually fighting in Scandinavia? Internal wars could have an effect upon the population pressures.
Could earlier conversions to Christianity come into play?
What could happen where Russia isn't, and in England, er, not-Normandy, and south Italy? Or is that getting ahead of myself? If I were to talk about the spread of Islam to the Slavs via the Volga trade route, now then I really would be doing that...