For me the real problem with Vincent is not that he is SOnny's bastard son, but it is the way he rises through the ranks so quickly. In the beginning of the movie he is nothing more than a tough street hood with no manners, no finesse, and little or no education. By the time the movie ends, Michael has given him about three lectures about keeping his temper under control, and all of a sudden he commands so much respect that his "betrayal" impresses Don Luchese. He seems to take over all security for the family (we see how THAT turns out) and he is a financial whiz to boot cause he comprehends the whole immobiliare deal. It is just not believable.