I think it would have substantially altered our present day because the Vikings were farmers. Whilst the stereotype is a pillaging marauder the viking people had farms and agriculture. Indeed, that's supposedly why "Greenland" was so inviting - it was billed as open, fertile land. By the time Europeans started sending colonists several native American tribes were already showing the beginnings of shifting to an agricultural basis. An established and successful farming civilisation, aware of agricultural techniques and the then-modern technologies , I think would have accelerated this dramatically. To the point that when European colonists started arriving they'd have encountered farming communities of native Americans. Farming communities might also have been less susceptible to the devastating plagues native American populations suffered shortly before the colonists arrivals. Farming communities would also have appreciated the concepts of land ownership in a way that the more free-roaming tribes did not. In short, I think European colonists would have not had such an easy time of it in a timeline where viking settlers had created an agricultural model for native Americans to adopt. The same would have been true if, say, the native Americans were just a couple of hundred years further on with adopting farming even if their technology was otherwise more or less the same.
Furthermore, whilst the Vikings aren't most known for cavalry, I think it's likely any colony in North America which maintained ties to its homeland would have tried to introduce horses at some point. Native Americans didn't have horses until the Spanish introduced them and even then they took quite a long time to spread Northwards. Imagine if instead of Native Americans being on foot when Europeans arrived, they had arrived to find native American cavalry. I don't know if the vikings would have led to an early adoption of the horse - I don't know enough about it but I figure it's a 50-50.
Last thing that occurs to me, and it's a big one, is if viking ocean-going ships were adopted by native Americans or even if just the possibility of long-distance trade led to further development of such technology just for the vikings and their allies. I agree that a Viking colony, even if you thought they would have the edge over native Americans militarily which I don't know either way, would not displace Native American tribes. So two outcomes follow from the development of improved ocean-going ships. One would be the Vikings themselves becoming a stronger power in Europe than they actually were and longer-lasting, going from raiders and sporadic settlers to real dominators or European seas. I wonder what a Galleon would look like if it had descended from Viking long-ships! Second outcome would be the possibility of Native Americans that adopted such ships becoming more of a seafaring people themselves. Imagine Native Americans arriving in North Africa in the 1400s for some serious Mohammedan on Geronimo action? Or good grief - imagine the Aztecs sailing up the coasts of Africa to raid and pillage or even visit Europe? At the very least, they'd make an incredible source of mercenary forces for whoever paid them. Though with the amount of gold and silver they had, they'd probably be successful traders for ages.
Why couldn't the vikings have stuck around in North America? It would be amazing!
