Cool little theory I had here:
Vincent died on the plane crash (or soon after), but the smoke monster saw he could take over Vincent's body and gather information and find a good person to do a loophole. Here's an example to back me up: When Shannon was chasing Vincent in the forest, he ran off and both he and Sayid disappeared in the jungle. Just then Walt was there, and we know the smoke monster was using his body. Also, it seems the dog is always gone at big events, and when the monster comes he isn't there either.
Just a theory :P