Michael said, "Nephew, call yourself Vincent Corleone." He didn't say, "call yourself Don Corleone," probably because Michael still regarded himself as the legitimate Don Corleone. Had Michael died and Vincent taken over, he probably would have called himself Don Corleone.
Carlo Gambino was known as "Don Carlo" by all. Castellano was known as "Paul," or "Paulie" by those reporting directly to him. I don't know what the lower-level people called him.


Ntra la porta tua lu sangu � sparsu,
E nun me mporta si ce muoru accisu...
E s'iddu muoru e vaju mparadisu
Si nun ce truovo a ttia, mancu ce trasu.