seem to have attacked before they thought about it in the new world- there is evidence that the skraelings that they attacked in the early days of the settlement may have come back to finish them off.
Perhaps it's not surprising, considering Erik the Red was a murderer, and the son of a murderer.

They did also trade with them however. I would think that the tribes they traded with were not the ones they fought with- intertribal friction was hardly unheard of, and different groupsd might have different ideas towards the red-haired giants.

What drove the people away from Vinland and why did they not thrive? That is indeed the question! Evidence is scant-sagas were written hundreds of years after they occurred, information gets lost. We do know the natives attacked the settlements and there was sure a lot more of them than the norse. We also know they tried to do things in the style of a Norse farmstead, which was not ideal for borderline conditions...but really, I don't know any better than you do...
BUT you gotta hear this theory. I happen to live in the area, so I like it...see what you think! http://forest.facts.tripod.com/xvinland.htm