I know that iceland was ,,fully settled,, by then. Ive seen figures of 75k, probably later. If iceland was only 25 k then, im not sure where there was room to triple the population. 10% WOULD be a lot to move, ill admit.
Still, younger sons wanting their own land, and often better land than what older brother inherits, would provide a fair impetus.
Iceland was "fully settled" by 930, and overpopulated by 976 or so (a famine is recorded then).
When I tried to guestimate plausible population numbers, say:
Quebec French multiplied from 60 000 in 1760 to 6 millions around 1960. 10 times increase per century.
12th century Norse do not have 19th century medicine or agricultural technology. Let´s say their natural increase is merely 4 times per century - doubling in 50 years.
Now, we know that Iceland of 986 had 1000 willing emigrants of the population of perhaps 30 000. 560 settled in Greenland, some others returned, the rest perished in storm. The godis did not prevent their landmenn from moving - whether they did not want, or could not.
In the 14 years that followed, some more shiploads settled in Greenland.
Would it be out of scale for Iceland and Greenland combined to provide 60...80 emigrants to Vinland per year on a sustained basis?
Say the total Norse population of Newfoundland is 2500 by 1050. And then each half century natural increase doubling of the existing population PLUS extra 5000 for the 3000...4000 new immigrants and their natural increase.
That would be:
10 000 by 1100 (2500 natural increase, 5000 new immigrants)
25 000 by 1150 (10 000 natural increase, 5000 new immigrants)
55 000 by 1200 (25 000 natural increase, 5000 new immigrants)
115 000 by 1250 (55 000 natural increase, 5000 new immigrants)
Beyond 1250... What might be the High Medieval population of fully settled Newfoundland? Much warmer and more fertile than Iceland... but High Medieval Norway is bigger, and had about 350 000.
I guess that after 1250, Newfoundland would get fully settled and start to be affected by overpopulation and famines. Population growth would continue in the (already explored and settled by pioneers) Quebec and Maritimes, and emigration from Newfoundland would grow, but since it involves distant trips and adaptation to somewhat different conditions, the growth of Norse population in Vinland would slow down.
But continue in long term throughout High Middle Ages.