Vito thought always that Santino would make a bad Don.His critisism against Santino is almost insaulting since the beginning of the movie.On the other hand,as made obvious in part II, Vito had a huge love for Michael.(tuo padre ti voule ben assai,as the train departs from Sicily).But he had forseen that the family's buisiness should change, that's why he didn;t want Michael involved. Santino couldn't realize that family would change their way of doing buisiness and tha's why he would make a bad Don according to Vito.

However,Santino was a perfect wartime Don. Michael never forgave Tom for his weekness during the times that Vito was out of action. Santino moved fast and critical,as he should have.He waited for the real intentions of the other families and when it turned out that their primary goal was to kill Vito,he acted. That's what forced Solozzo to talking with Michael.