Is a clue to the fact that forms are being replicated on the Island (by Jacob's enemy/the monster). In "So It Begins", the "Christian" talking to the dog is wearing both shoes, and this happens a mere minute or two before a groggy, awakened Jack runs past the single shoe stuck in the tree. This is a clue that the shoe in the tree is half of the actual pair that Jack put on his dead father's feet, while the shoes worn by the Christian "apparition" who addressed Vincent are a replication, as is that apparition itself. The real shoe jammed in the tree fell out of the real, dead Christian's coffin in the crash.
We don't necessarily know that definitely. His behaviour here seems to be out of the ordinary for MIB, as he shows a lot of concern for "his son". It's very different to when MIB appears as Yemi and impassively hears Eko's speech before making a point of saying he is not Eko's brother. It's possible that Christian himself was appearing somehow, especially because Vincent seems to be particularly perceptive and might be able to receive a message like that from a dead person