When things started to go bad, they were allready depending on Icelandic Merchants for trading purposes, not having the abilty to build the boats needed to get to Vinland
Not really... greenlanders were voyaging to vinland and markland right up until they vanished in OTL.
Again - Greenland did not fail so much as the people did not want to live in such a cold, dark climate on the edge of civilization. Eventually they gave up and headed for warmer pastures (where is still in question, for they did not flee to iceland or norway or the british isles... but they did go somewhere).
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You want Greenland to prosper and/or Vinland to expand in OTL? Keep them pagan (or as a safe haven for pagans). Christianity really took a lot of the conquering/colonization spirit out of the Norse.