I like the character of Vincent, but I don't necessarily think he'd make the best Don.

Vincent has Sonny's charisma and Vito's compassion for the family but he isn't the level headed thinker that Vito or Michael was.

He seems to act far too rashly (just like Sonny did). His devotion to the family is without question and he is the kind of guy you'd rather have on your side than against you but as a Don he probably wouldn't measure up to Michael or Vito. Sonny loved his family too but he made some bad decisions when he took over while Vito was recovering.