For whatever reason I decided to check out Vincent's page. Operating off of MIB's statement that the reason he showed up to Jack as his dad was to lead him to water. I'll buy MIB's explanation a little. With this in mind, considering all the travels and revelations Vincent provoked, maybe MIB (if not Jacob), appeared as Vincent to lead people in certain directions, too? Speaking of all this. MIB must have shown himself as Walt, too, to account for Shannon's experiences of seeing him.
Another totally tangent detail. Reading the page reminded me that Sayid had a thing with Shannon in the first season, didn't he? This put some tension between he and another islander. Just a little interesting detail that a dark-haired Nadia wasn't the only love of his life. He was having some pretty strong feelings develop for someone else entirely different.