This seems like it should be a very simple question but for some reason absolutely no source I have been able to find addresses this... if Leif Erikson discovered America before Columbus, then why does nobody in Europe seem to know about America until Columbus set foot on the Caribbean? Why didn't Leif Erikson tell anyone, and, since he did seem to keep it a secret, how did we even find out that he discovered it first?
I've answered related questions here and here and u/Platypuskeeper has talked about other historiographical debates about Vinland here.
The most important takeaway is that Leifr, or Thorfinnr karlsefni for the later voyages, did not keep Vinland a secret. Adam of Bremen knew about it in 1070, Icelandic annals report an attempted voyage by the Bishop of Greenland in 1121, and the two sagas that tell of Leifr's voyages were composed in the 1200 (and written down in the 1300s!) Additionally, by the 1600s, there is a lively debate over whether Vinland was 1) real and 2) in North America, and if so, where precisely the landing site was.
You might want to check out some of the posts in the European exploration and colonialism section of the FAQ that explore how the discovery wasn't forgotten, exactly, but not put into the context of "this is a whole new content".