Yes TB, Vito is a composite figure. Regarding your comparison of Vito to Costello though keep in mind that Vito's effort to pacify the underworld consumed only a few years of his tenure as Don whereas Costello's efforts spanned a substantial part of his tenure as Don. So, I think that comparison is cursory. However, dominating Vito's entire tenure is his familial relationships. Those relationships most favorably compare with Bonanno's.

Also, I would think that anyone who selects a godparent for their children would select someone with whom they have a close relationship. So, that's a given. As I've stated above, Altobello is the Corleone family's oldest friend. There's no indication, implied or inferred, that he is or was a Corleone family member. And while Don may be a term of respect, in the Trilogy and novel it is not used in any other context except as applied to the head of a crime family. In other words, GFIII does not present Altobello in any subsidiary role.