I think toward the end of GF III, the character of Vincent began to come along a little. In the first half of the movie, he's really little more than a cariacature of Sonny.

(Basically, as soon as they let the icky-poo semi-incest story line take a back seat to the Immobliare maneuverings, and let Vincent actually start taking more of an active role in the decision making, the character starts showing some depth.)