Please remember that the book and the movie are two different things. Dr. Jules Segal doesn't exist in the movie. There is no reason to believe, in that context, that Lucy didn't give birth to Sonny's child. Everyone seems shocked that this character was introduced, since he never existed in the book, but neither did Mary Corleone, in the book Kay left Michael, which doesn't happen in the movie. There are many differences between the novel and the movie. At least Vincent's existence is possible, given that difference.