Vincent didn't have much calculating to do, Micheal gave him the "order" because there was no one else around, anywhere. I see Sonny in Vincent, and the only difference is that Sonny had a better teacher then Vincent and was smarter(fate notwithstanding Vincent was clearly going down the same road as well, or maybe did)then Vincent. As for the warmth factor... i see Fredo as stupid and selfish no "warmth" there, he's helpless, and clueless not "warm", Vincent could have gotten some of that. As for Micheal i saw no Micheal in Vincent, Vincent was stupid and reasonless for the most part,and it was very hard for him to let Mary go as his face suggest where as Micheal would have done it w/o even showing emotions. He maybe had a little Vito in him because he had "the Corleone courage" but then again, Vincent was stupid so somethings he did might have come out of his stupidity.

Vincent is his own character, 2D womanizer, i honestly couldn't see him with a family if he was in GFIV and if the movie did ever come out. We will have to wait and see, any of the qualities of the three come are made more conspicious in the 4th movie because i could in no way see them in the 3rd movie.