Vincent was a fabricated character who, if you read the book, never existed and shouldn't have existed in the film. While you can extrapolate and justify Frank Pantangelli, and some of the other characters of GFII, there's no way on earth you can do that with Vincent. He was created out of expediency, and Coppola's willingness to desecrate the nuclear family he and Mario Puzo worked so hard to create is shameful.

Therefore, I do not even like to ACKNOWLEDGE his existence, even in fiction. And, ergo, I cannot comment, nor do I WANT to comment, on his personality development, such as it is.