Alt. Hist. request, Vikings make a go of the "New World"?

[Minimalist]

This is interesting, timing-wise.

https://www.jpl.nasa.gov/news/news.php?feature=5613

  A giant volcanic eruption in the South Pacific in the 15th century that until recently was known only in island legends has been confirmed by a Jet Propulsion Laboratory scientist.

Using clues as diverse as growth rings visible in the wood frames of British portraits and crop records from China, Pang fixed the year of 1453 as when the legendary volcanic explosion must have taken place.

Pang said such an eruption, equivalent to two million Hiroshima-type atomic bombs, would have changed the world's climate. His research, which delved into records around the world, proved that to be true.
In his study, Pang drew on evidence found in tree rings, ice cores and in the historic records of civilizations in Europe and China. Oak panels of contemporary British portraits had abnormally narrow rings in 1453-55.
In Sweden, corn tithes fell to zero as the crops failed; western U.S. bristlecone pines show frost damage in 1453; and the growth of European and Chinese trees was stunted in 1453-57.

How much deterioration of the climate would it have taken to make Greenland untenable?