If I remember correctly from the novel, Vito and Micheal were also planning the "baptism" hit on the Five Families during the time fo Micheal's "training" for the Donship. Maybe there were some details of this plan that hadn't been covered, and Micheal was worried?

This topic raises another question: how exactley did Vito "transfer" his political contacts (his "nickels and dimes) to Micheal? It's not like transferring property, or even regimes. Did they just invite all the senators, judges and politicians round for dinner and casually announce that, in the future, Micheal would be the one to go to for "favours"?