I think some people are misinterpreting the most recent mobisode from LOST: Missing Pieces, entitled “So It Begins“, which takes us back to the moment just before the start of the pilot episode in season 1. It shows Christian Shepherd and Vincent talking to one another, and Christians saying Jack “has work to do”. But that is just what Walt says to Locke in the season finale, and there one assumes that it is the island communicating with Locke, and not that Walt has in fact returned. Or perhaps it is Walt communicating from wherever he is – that too would be a possibility. But since Vincent has at times appeared to be a manifestation of the island (or of the smoke monster, which may or may not be the same thing), why cannot the conversation between Christian and Vincent be such a manifestation as well?
LOST: So It Begins