Vinland's location (realistically, somewhere in OTL southern Acadia or northeast US) is not the problem. The problem is they are not that much more advanced then their neighbours - sure iron gives them a big advantage (and they have near complete control of the seas), but its not the same as the gunpowder advantage. IMHO - the best bet for a surviving Vinland is for the colony to be somewhere more northernly, like OTL prince edward's island. It's small enough that they can control it and hold it, and then expand from there. One thing most people don't realize is that the greenland during the era of 1000-1200 was one of the healthiest places to live in europe, mainly due to it's great plethora of food (ironically). So once a location in Vinland is secured, there would be a steady increase in settlers arriving and populations would expand rather quickly imho.
cheers.
cheers.