If your gonna talk bout Vincent then lets talk bout Mary.

She wasn't even in the orginal book Mike and Kay had two sons in The Godfather then in the movie part 2 he all of a sudden had a boy and a girl and in GF returns same thing.

And now lets talk bout the baptism from GF it wasn't in the book it was a confirmation two completely different things so if your going to think about that one little difference(Vincent) then think bout all these other ones the book and movie are two completely different things Cuneo and Stracci didn't die in the book so why did they die in the movie?