I'd like to pimp my own TL on that,
https://www.alternatehistory.com/discussion/showthread.php?t=34545&highlight=Vinland+bountiful
which I will return to, now that there is less pressure on my work/study combination.
If the Vikings land in Vinland and establish colonies there, would it be possible to keep them up there? If the Vikings spread stories about Golden cities in the Americas, that would attract even more people than OTL.
Would it be plausible to have long-lasting Viking colonies in Vinland that do not establish any sort of contact to native tribes in the Caribean?
And how fast could plagues spread from Vinland through America?
Very quickly. Not quite as quickly as the Black Death in europe, which took advantage of shipping, but as quickly as the plague spread from Asia to europe.
It would be possible to keep the vikings there. All you really need is a less halfhearted attempt, and a better, defensible location. As I recall, in OTL, it was tried by a couple of families in the cold north. A larger group, setteling a bit futher south, perhaps on an wooded island some distance away from the coast, and close to good fishing banks should survive quite well.
There would also be a good window for expansion and entrencing when the diseases hit the natives.
However, just like Greenland, the little ice age would cut them off from Europe at some point.
It is also quite possible to have a colony that does not have (direct) contact with the Caribbean islands. While I am quite certain that someone would go exploring down the coast of Vinland, would the Caribbeans offer anything to keep them coming back?
It depends a lot on how well the colony does, and how much surplus there is for young men to go a-viking. A small colony could find itself stymied by hostile natives, and just hanging on, as said, with no opportunity for voyages much futher than, say, Georgia.
A colony with passable relations with the natives, could expand and develop far greater population density than the natives, also taking immigrants from Greenland as the climate worsened. They might well go into the Gulf of Mexico, and to the Yucatan, up the mississippi, and to the great lakes. The last, I think, absolutely requires good relations with the natives.
I don't know the answer to this, but to any (professional or armchair) biologist who wants to venture a guess, I'll add this question -- could a plague come too early? In other words, if the Romans or Egyptians landed in 1 AD and never, ever returned, would the diseases self-perpetuate, or would the very rural nature of most of the Americas mean that the diseases would eventually disappear, and within a generation, the immunities would disappear, too?
It depends on a lot of factors I don't think we know. The population density at the time of the contact, and at the point of the contact. The speed of communications and travel in the Americas at the time.
It would also be possible for the plagues to be so virulent that they failed to spread if they were introduced to a small group. If two or more diseases hit, a caribbean island, for example, offers a good chance that all the natives would die befor contacting any other natives.
But at a guess, there is a also a chance they could end up self-perpetuating.