Agreed. This is ASB. Even if no one from continental Europe discovers the Americas, it wouldn't be that hard to establish a colony of Greenland Vikings somewhere on the east coast of North America.
Not necessarily. Greenland Vikings could hardly stay on Greenland and the rest of North America. Even the first European settlements had a hard time adjusting. A lot of them relied on being on good terms with the natives; the Vikings didn't quite seem so inclined to that, especially after their encounters with skraelings.
All you'd need really is a butterfly or two. True European oceanfaring technology only really came about after John I of Portugal and his Illustrious Generation (mostly Henry, but butterflying Henry away could just lead to one of his brothers filling his role)'s interest in oceanic exploration, the latter fueled by searches for Prester John and the source of African gold.
If you find a way to halt that instance of advancement of maritime technology (and preferably interest in it in the first place),you could potentially buy the Americas a lot of time. Can't sail west to the Orient if your best ships are too heavy and slow, and proposing to spend lots of money to create a new type of ship for an experimental trade route would be out of the question.
Alternatively, by the time Columbus was proposing circumnavigating the globe, Bartolomeu Diaz discovered a viable trade route by sailing around Africa, which seemed to be a very practical method and made it hard for Columbus to convince people to fund him. This combined with the fact that Columbus' figures for distances and logistics were far too low and looked down upon as unscientific, and took him two years in Spain to actually make Ferdinand and Isabella consider funding him, and the only reason they kept him around that long was because they didn't want him (successfully) selling his idea to other people. You can tip the balance by having general opinion being Diaz' route by far the most practical solution to reaching Asia, and emphasizing the unscientificness of Columbus' proposal to the point to where the two Spanish monarchs don't see any economical or political danger to letting him run off elsewhere. Columbus fades into history as a guy whose ideas were interesting, though immensely stupid (and in modern times perhaps looked back upon and considered that it just might have worked, sort of), and with little incentive to sail west due to the popularity of Diaz's Africa route, the Old World remains ignorant of the new and still considers it an untraversable massive ocean for a good long while, until economic conditions shift and someone needs to find an alternative route -again-, at least.
Speaking of Columbus, the fact that he managed to make the return trip at all is complete pure dumb luck. First off, he sails the Atlantic right in the middle of hurricane season, and super close to the horse latitudes and potential tropical storms. There was plenty possibility for him to get either stuck or sunk. Returning from the Caribbean he decides to use knowledge from the Portuguese's volta do mar (or makes this decision on a hunch) and sails
north into the mid-Atlantic, miraculously catches a source of wind going east and back into the tropical storm and hurricane zones. He basically had lots of opportunities for pretty much not coming back at all. Had he lost his ships, he would be remembered as the man with poorly educated ideas who led his men to certain death (
"We tried to tell him that you couldn't traverse 7,000+ miles of ocean without running out of food and fresh water, but the poor bastard wouldn't listen"), and could serve to definitely scare Europe from ever trying anything similar until the proper technologies are refined in the areas of onboard food production (like Chinese exploration ships), desalination (Ancient Greeks had desalination devices, just refine them for use by large ships with lots of people and animals) and rainwater storage, and advanced maritime logistics and science. All of this would need both time and the incentive to develop.
And then there's that almost mythical story about Columbus' crew conspiring to mutiny to save their skins. Though I'm not sure about the historical accuracy of that, that could work too to accomplish the effect of the previous paragraph.
Really, there's quite a number of ways you could do this.