Personally, I believe that if Scandinavian settlements were to continue to survive in North America, Missionaries wouldn't be far behind as this is the era where Norway and Iceland would convert to Christianity. So rather than seeing Indigenous beliefs merging or competing with Scandinavian paganism, I believe that you would see Christianity absorbing local legends and beliefs. Certain spirits would become saints or angels, or demons. There could be war, but the settlers are also going to trade with the natives. You could see iron weapons being traded for beads or copper, or furs, etc. Or food. Religion would also follow these trade routes, and you could see Christianity potentially spreading through the great lakes region (seems a good place for Vikings to sail to).