Vinland also lacked a nobility below Leif, there was no Serfdom in Vinland, people lived on farmsteads that they owned themselves. They still had to pay a tribute to their lord for protection but that lord was in Leifsbuðir, and getting the money was occasionally quite difficult.
I expect that there WOULD be other nobles.
Look at Iceland. Ingolfur Arnarsson was the First Settler of Iceland. He and his son Thorstein dominated their immediate neighbours and Kjalarnes thing. But they could not effectively prevent other nobles from sailing their own ships to Iceland, circumnavigating the island and settling away from Ingolfur.
When Althing was founded in 930, the organizers of the union found 435 first settlers, and designated 36 chieftains to rule Iceland. (They were slightly wrong - a few decades later they had to recognize 3 more chieftains they had initially ignored). They recognized Thorstein and his descendants as heirs of the First Settler, with title "allsherjargodi", and accorded them the hereditary honour of opening each Althing session - but the presiding official, the Lawspeaker, was to be elected.
The result?
The heirs of Ingolfur were minor chieftains, despite holding the symbolic office. Only 1 of them - Thorkell mani Thorsteinsson, grandson of Ingolfur - was elected as Lawspeaker, and that in 970, 25 years after inheriting chieftainship. Later on, the office of allherjarsgodi continued to exist but was regarded as so unimportant that the thorough Icelandic Sagas do not bother to mention who held the post between 1055 and 1160, or after 1234.
When important chieftains, called storgodi, did start to arise in Iceland, starting with second half of 11th century, it was different families, beginning with Haukadalur family.
Now, Leif CANNOT actually stop other noble chieftains from settling elsewhere along the coasts of Newfoundland or Labrador - like Ingolfur could not.
But he does have some advantages Ingolfur, Thorstein and Thorkell did not have. (BTW, for reference where diplomatic relationships are concerned: the post of allherjarsgodi was held by Thormodr Thorkellsson from 984 to 1020; the lawspeakers were Thorgeir Thorkellsson Ljosvetningagodi, 985-1001, Grimur Svertingsson, 1002-1003, and Skapti Thoroddsson, 1004-1030).
For one, the immigrants are not coming so rapidly. 11th century Vinland is much further from Norway or Scotland than 9th century Iceland was, so fewer potential emigrants can afford the trip. Iceland and Greenland are closer, but there are rather fewer people there, and fewer immigrants coming. While chieftains do settle in Vinland away from Leif´s Leifsbudir, and they are asserting their own noble status, they are trying to keep good relationships with Leif unless provoked. And Leif tries to avoid antagonizing them - inviting them promptly to Vinland Althing, conveniently located near Leifsbudir.
For another, unlike the completely unsettled Iceland and Greenland where an immigrand had no natives to deal with, Vinland is settled by Skraelings. Any new immigrant chiefs from Iceland, Greenland or Europe are advised to pay a visit to a friend already settled in Vinland, take some Skraeling language interpreters and guides to help choose a spot on the coast and negotiate with Skraeling neighbours. And that settled friend with guides to spare is often Leif.
Also, trade. What made Haukadalur of Iceland prominent in late 11th century included the fact that Southern Iceland is the one region where trade cannot take place all along settlements - there are large inland plains and sandy shorelines offering poor harbout, so trade was concentrated at Eyrarbakki. That was foreign, mainly Norwegian ships visiting Iceland - Icelanders could not build ships because local timber was not good, and they imported few ships. Whereas in Vinland, any independent chieftain might build his own ship and sail to Greenland, Iceland or Europe to trade.
Might - but this can be unwise and risky. It still means mobilizing tens of men for crew, and being away from home for a year. And the ship might be wrecked.
Leifsbudir is convenient for communications in a way that neither Reykjavik nor Thingvellir is, nor Brattahlid. For the Belle Isle Strait brings together 4 coasts - the Atlantic and Saint Lawrence Gulf coasts of Newfoundland and Labrador.
Any chieftain in Vinland with pretensions to wealth and nobility can just build a small sailing ship - not a full sized knarr like Skuldelev 1 but a smaller one like Skuldelev 3 byrding - take a few crew and go to Leifsbudir to trade and be back in a few weeks. Leif, in his turn, is the only chieftain in Vinland who could afford to build and man not one but a few knarrs, hire captains and send them to Europe in turns. And eat the losses if one of his several ships is lost.
If Leif takes care to keep his profit margins modest then the other chieftains will not bother trying to compete with him. And Leif is not offending them by enacting his legal monopoly of trade or charging tolls - if a few ships sail elsewhere in Vinland independent of him, let them take their losses.
How about sustaining this kind of domination throughout 11th century?
