Quoting myself to put these forward (big snp)
Exellent! My two cents on each...
POD 1 - Laki type eruption in Iceland forcing the population to flee.
I'd add "definite worldwide effects" and "possible loss of Iceland as a stepping stone/supplier" to the list of Cons.
As for "worldwide effects", during the Iceland ash problems earlier this year, I read a BBC article about the Laki eruptions of 1783. Researchers looking into UK parish records had come to the realization that a large spike in the rural death rate occurred in step with the eruptions. They estimated thousands, and maybe tens of thousands, of deaths could linked to atmospheric pollutants released by Laki.
As for "loss of Iceland", the Norse aren't going to sale directly between Scandinavia and Vinland or receive needed supplies directly from Scandinavia either. Iceland is going to be definitely needed, especially during the initial settlement's early years.
POD 2 - charismatic leader step forward.
Too much of a "coin flip" for my liking. And BEHOLD Shemp the Magnificent was born and, unlike anyone else is history, he led everyone to Vinland for reasons that he, unlike anyone else in history, could alone comprehend..." Might as just make a single person ISOT and get it over with.
POD 3 - Olaf the Holy survives Svolder but is defeated.
This is the one I'd vote for if we were voting. First, the POD is "large" enough to accomplish what we want while staying "small" enough to avoid consequences which would completely overshadow what we want. Second, it's a known historical person with a known behavior and a known agenda so his post-POD actions remain plausible.
Putting it another way, Olaf's survival won't have immediate or short term consequences in Europe or elsewhere, but his survival could very well spark a slow exodus from Iceland to an already known Vinland.
POD 4 - discovery of Grand Banks of Norwegian fishermen sometime after discovery of Vinland.
This one deserves some attention because salted fish could become a trade item. That trade would require better ships for the Norse and a large enough market in Europe. The Basque were able to sail directly between Iberia and the Grand Banks, something the Norse wouldn't be able to do. The Basque were also supplying a market which was rich enough to pay for fish, a market whose fish stocks were already showing signs of depletion, and a market create by an almost continent-spanning religious requirement.
We know the Norse didn't have the ships and, in ~1000-1100 CE, I'd say no for the three "market" requirements too.
So, is it Olaf?