I'll split this into two questions: 1) How did the Norse interpret their discovery of North America? and 2) How did they disseminate this knowledge?
The Norse did not realize that they had discovered a vast continent. The first Norseman to report land west of Greenland (Bjarni Herjólfsson) reported three different regions: Helluland, Markland, and Vinland. Modern historians generally agree that Helluland corresponds to Baffin Island, Markland to Labrador, and Vinland to Newfoundland. Norse settlers mostly ignored Helluland, which had little in the way of natural resources to offer. They valued Markland as a source of timber (and may have been traveling there as late as the fourteenth century), but did not settle. Vinland was the only site of long-term settlement, but this experiment ended prematurely for somewhat unclear reasons. There was evidently not enough motivation to continue the effort, so Norse knowledge of the region remained fairly limited in the following centuries. The Norse never realized that the lands they had explored belonged to a massive continent. They appear to have thought of Helluland, Markland, and Vinland as remote islands off the coast of Greenland.
The Norse transmitted knowledge of these discoveries orally for three centuries before putting them to writing. The two main accounts of the North American expeditions are the Grœnlendinga saga ("Saga of the Greenlanders") and the Eiríks saga rauða ("Saga of Erik the Red"), both of which were only written down in the thirteenth century (and even then, not in a language that most Europeans would have understood). So the Vikings didn't "tell nobody," they simply described their voyages to a very limited audience.
There is, however, one instance in which a non-Scandinavian reported the Norse discovery of Vinland. Adam of Bremen was a German chronicler who wrote the Gesta Hammaburgensis ecclesiae pontificum ("Deeds of the Bishops of Hamburg") in the 1070s, several decades after the initial Norse discovery of North America. His book contains an extended ethnography of Scandinavia, and contains this report:
"[The King of Denmark Svend Estridson] has also reported one island discovered by many in that ocean, which is called Winland, for the reason that grapevines grow there by themselves, producing the best wine. Unsown crops also abound on that island, as we have ascertained not from fabulous reports but from the trustworthy relation of the Danes." -- Gesta 4.38
So Adam, an educated Latin chronicler, evidently took the Danes at their word when they described Vinland as an island. This was interesting, to be sure, but not revelatory -- so it's no surprise that this small report of a previously unknown island in the Atlantic attracted little attention. In any case, Adam did not personally think the matter worth investigating further:
"Every place beyond it [Vinland] is full of impenetrable ice and intense darkness . . .The very well-informed prince of the Norwegians, Harold, lately attempted this sea. After he had explored the expanse of the Northern Ocean in his ships, there lay before their eyes at length the darksome bounds of a failing world, and by retracing his steps he barely escaped in safety the vast pit of the abyss." -- Gesta 4.39
In other words, the discovery of Vinland did not greatly alter European notions of the Ocean that had persisted since antiquity. A remote northern island might be of interest to adventurous Norse sailors, but it was only of passing curiosity to someone like Adam of Bremen, even less so to the average European of the time.
In conclusion, the Norse never explored North America thoroughly enough to know that they had stumbled upon a new continent. They considered Vinland a remote island off the coast of Greenland. Since this knowledge was preserved for centuries only as an oral tradition (and later as a text in an obscure vernacular language), not many people outside Scandinavia ever heard of Vinland. Those that did (i.e. Adam of Bremen) generally reported the discovery exactly as they had heard it from their source, referring to Vinland as a remote island at the edge of the endless Ocean. Only in retrospect (and with the handy help of archaeology) have we realized that Vinland was in fact North America -- and this realization did not happen until the 19th century!