I think Vincent would have been a good Don.

Although I think ‘they’ weren’t going to make it easy on him… so he was probably going to be forced to ‘prove’ himself, maybe even in the way Michael did in GFI.

The fact that Michael found peace in Sicily tells me he had faith in Vincent and therefore ‘the family’ didn’t need his protection anymore. I don’t believe he would allow quarrels between ‘brothers’. The Corleone’s are Sicilian and therefore as Michael had appointed Vincent as his follower that decision was to be respected.

But… when Michael dies… then the trouble starts for Vincent. Other families start questioning him… after all he is a ‘bastardo’. Vincent was tutored well; Michael probably gave him Vito’s legacy, the wisdom, the vigilance, the brilliance of outsmarting…

Vincent was probably going to have a hard time, maybe even people in his surroundings would betray him… men he doubted showed to be faithful… and the game was also becoming more complicated and dangerous as other nations become powerful not to forget that Narcotics was going to reign the ‘Underworld’.
But I think as he is a Corleone – he was to survive and live up the Corleone reputation.

Was he going to live long enough to become old as well? I don’t know. But if he uses the knowledge that was given to him… yes…. I believe he would make it… who knows… maybe even have same ambitions as Michael – make the ‘family businesses legitimate.