Based on the movie and the novel, we know that Vito Corleone thought that "Santino was a bad don rest in peace." He lacked subtlety and lost his temper which led to his downfall. Also, during the Sollozzo meeting, Sonny made an almost fatal mistake which gave Sollozzo an opening by showing interest in the drug deal. In the book, they mentioned that Santino did win some major victories in a gang war in his youth.

Would Vincent have made a better don than his father? Obviously, he learned a lot from Michael but would it be enough? Supposedly, in the rumored script for the Godfather Part IV, he would lead the family back to the world of crime and lose his life at the end.

I think Vincent would be slightly better than Sonny as he really listened to Michael whereas Sonny didn't learn as much from Vito and his temper was a major weakness.