Byzantine Empire invades Vinland

Hmm. Not much of a one, I would think. Meditteranean naval technology really wasn't suited to the conditions of the Atlantic at that time. A light breeze on Spain's Atlantic coast would be about enough to put a few hundred dromons at the bottom in no time. They might have been able to use some roundships as coasters in the Atlantic, but these weren't suited for war and still wouldn't have lasted 5 minutes out of sight of shore. So, unless they imported some Saxon or Scandinavian shipbuilders using the northern techniques, they'd not get far enough to settle anything on the Atlantic.

Meditteranean craft like the dromon were behemoths of war compared to any of the longships etc though. The longships were fast, easily constructed, sophisticated, and able to capably handle the conditions of the open sea but they were not built for actual naval battle. They were meant to transport men in lightning raids, not to actually fight on the water.