Sorry but Norway's weakness lies in it's Demographics. No Amount of trade will fix that. Its either Denmark-Norway or Sweden-Norway.
Conquests can.
And Vinland is better placed to defend itself against conquests than OTL Iceland was. And has the interest and means to defend Iceland, too.
Not too sure on the need for European slaves. I suppose theres a chance that the Norse could get sick of Native slaves constantly dying like the Spanish OTL. Perhaps initially when settlement is confined to Newfoundland and the some 500 or so Beothuk populations are spread out pretty wide it may seem like a good idea to bring European slaves especially if your a Greenlander who doesn't have the faintest clue how to grow wheat.
Greenlanders DID know how to grow barley. Archeological evidence. Finds of barley, uncleaned so clearly local harvest not import. At least sometimes they got crops.
Newfoundland would be warmer than Greenland and good for reliable barley crops. The tiny patches of fields and meadows at the heads of fiords of southern Greenland could support perhaps 3000...4000 medieval people. Iceland could support maybe 30 000 medieval people and definitely did grow crops.
Gunnar Hamundarson said:
How lovely the slopes are, more lovely than they have ever seemed to me before, golden cornfields and new-mown hay.
Ireland is smaller than Iceland - and had over a million people in Middle Ages.
Newfoundland is bigger that Iceland or Ireland, less mountainous than Iceland or Ireland and warmer than Iceland but colder than Ireland. It seems a reasonable guess that high medieval Norse barley growers and cow and sheep grazers could keep 100 000 people on Newfoundland.
If Norse come to Vinland to grow barley then Newfoundland is warm enough for them. And the first 3000 or even 30 000 Norse on Newfoundland won´t be short of land either. Neither are urgent reasons to move beyond Newfoundland to Saint Lawrence or Maritimes.
The Norse surely would explore well beyond their settlements. (Erik the Red with his one ship and three years of exile explored Greenland coast all the way to Disko Island). But if they settle far south of their compact settlement, they are looking for something else than barley.
When they sail far enough, they will find maize and people who know how to grow maize. Somewhere around Montreal and Boston.
But that´s the northern limit, where maize grows poorly. And it won´t grow at all further north.
Barley could be grown further south, but it does not give much advantage over Newfoundland. So what can the Norse find between Newfoundland and the maize growers of Montreal and Boston?
Newfoundland is too cold for wheat. Wheat might be grown in Maritimes, but why settle there in the first place so long as Newfoundland is enough?
Mind you, when Norse do settle at Montreal, wheat would grow there. So would barley - but wheat would be useful for diversification.
Then the Norse would indeed benefit from importing wheat and Irish (slave or free) to teach how to grow it.