I posted a topic similar to this one earlier, Don Pappo Napolitano. I don't think the character "Vincent Mancini" should have been created by Francis Ford Coppola and Mario Puzo. Vincent was about 25(+)-years old in The Godfather Part III, so he would have been born in the mid-1950's, and there is no evidence of his existence during Lucy Mancini's relationship with Dr. Jules Segal in the mid-1950's (in The Godfather the novel), and Sonny Corleone died in the late-1940's, so I don't think Vincent was actually Sonny's son. Either Lucy lied to Vincent for more than 25 years, or Francis Ford Coppola and Mario Puzo made a critical mistake in creating his character.