I think I remember reading that Vito was made in the Bonanno family following the 3 captains hit. After Sciascia was murdered he was invited to NYC by Massino to be bumped up to captain of the montreal "crew".

Obviously Vito never took him up on that.

In any case I think the reality here is that the Rizzutos had familial ties to the Agrigento clans and traditional ties to the Bonannos. It seems that they only paid lip-service to those traditional ties, but it would have been the official "running story" I'm sure, that, on the books, they are a crew of the Bonannos. It could be the Buscetta simply didn't know any more than that.