Vincent Mancini is the illegitimate son of Sonny Corleone. He is very eager to be part of family business commanded still by Michael Corleone (Al Pacino). Garcia in his performance attempts to create a character that is basically a combination of Sonny, and Micheal. He is Sonny with a inherited temper as well as the eagerness to join the family, but at the same time like Michael in having greater calculation and ambition as his character progresses.
Garcia in his representation of his inherited qualities from Sonny does not quite match up with the great performance given by James Caan in the original film. The simple truth is Garcia just does not have the same intensity that really seared off the screen as Caan did. Garcia just does not have the same visceral ending, but than again maybe he does not have to have it exactly because he is hot headed but not nearly as hot headed as Sonny was.
Garcia really though still manages well enough to bring Vincent's over ambition to life as well as do it with certain degree of charisma, when channeling his side that stands closer to Michael. Garcia's performance in this aspect also hardly comes close to Michael's performance in either of the two previous films, but than again Vincent is suppose to be similar but not nearly extreme, so Garcia really did not have to be on the same level as Pacino to begin with.
Besides his growth within the family business an important aspect of his performance is Vincent relationship with his cousin Mary Corleone (Sofia Coppola). The relationship is quite awkward and not so much because they are cousins in the film, but far more so because Coppola is really really bad with her Razzie wins well earned. The relationship does not work because Vincent is suppose to really love her, but Coppola fails to show any reason why he should. Garcia tries his best, he really does, but due to Coppola he can't bring anything out of the relationship.
Garcia's whole performance really is just fine he appropriately portrays Vincent well enough, but than again he never gives a truly great performance either like his Godfather predecessors did. Garcia though never ever is bad even when with Coppola and still manages to give a fairly effective performance, it just never becomes a truly powerful or unforgettable work like Caan's and Pacino's work in the original film.