Perhaps discovery of America should take place before the year 1000 C.E. Maybe Greenland is totally bypassed by either Erik "the Red" Thorvadson, or by Ulf Gunnbjarnson, the sailor whom actually discovered Greenland. Even then, you might need somethig to encourage a solid amount of emigration from Iceland and the Scandinavian countries.
Lets say the aggressive Christianization of Olaf Tryggvason in Norway causes alot of Norse Polytheists to flee first to Iceland, until King Olaf, with the sons of the major Icelandic families as hostages, and pressuring the Althing to make Christianity mandatory, encourages alot of people to make the trip there. With their knowledge of the Norse colony in America, and their dealings with it's merchants that sail back to Iceland every Spring and Summer, often a good time of the year for cross Atlantic voyages, to sell timber and skins in exchange for livestock, crop seeds, and even iron-ore, America would sound like an attractive option for many people fleeing religious persecution or the centralization of royal power in the Scandinavian countries.
Even with a steady influx of Norse exodites, the initial colonization will be pretty well below the numbers of European settlers that came to America in the fiftenth and sixteenth centuries. And the fact that this all began with a private venture means that if this colony is a refuge for fleeing Norse Pagans, then royal backing is the last thing they need.
Enslavement of natives might potentially boost the colony's population in the long-run, but they had best choose their battles.
Iron ore will be found eventually as long as the American Norse prevail, and their superior science of metallurgy and ship engineering will make up for their relatively small numbers.
Horses, even the small ponies of Iceland or the Shetlands (and whose to say that larger Frankish breeds wouldn't have been sold in either Norway or Iceland), would be an important assett for the American Norse, for war, agricultural, or transport. If used conservatively in the initial decades of colonization, could become something worth trading with the native peoples. That also goes for cattle, sheep, goats, and pigs.
The Norse colony had best remain politically unified, to remain strong in the face of the native American tribes. Trade down the east coast, through the St Lawrence River, and into the Great Lakes region, would be helpful.