I think the primary difference as well is Vito has in his childhood grown up with the Mafia and how a dishonourable approach to an "honorable" system will lead to greater suffering. He as a result has A) accepted the life for what it can be and B) has injected his own spin on the "life", seen the inherent good that go along with the system, the honor, the freedom. Vito in essence is able by his acceptance of one extreme of the "life" able to take a new spin on the whole system. He still is a criminal but it's my contention that FFC and MP have focused on his positive contributions due to the fact that he is noble regardless of the life he leads.

Mike on the other hand was an outsider thrust into a world he did know about, but he still wasn't fully submerged in. He thus perhaps lost the ultimate teaching of Vito. Be reasonable, this is shown in GF II to he his one short falling. He killed Fredo, Roth, all these people who in my opinion were very weak and punished by their crimes. Roth was in custody, Pentangeli a prisoner, Fredo, ousted from the family. They all were punished, Mike took them all out based on principle, not reasonable.