Mmmm... I think Vito wanted Michael to be the "legal" one in the family, because he confesses to him, "I never wanted this for you", when Michael already was in charge of the business. I think it's spelled "pezzonovanta"(and I'd really like to know what it means in Italian... does anyone know?).
"...Luca Brasi held a gun to his head and my father assured him that either his brains or his signature would be on the contract. That's a true story. That's my family, Kay. It's not me." (Michael Corleone)