Would Vito still be able to plot revenge?

If Michael was killed in Italy during his hide out, and Sonny was already assassinated.

How do you think Vito would play this one out? Would he still have the strength to devise the plan to wipe out the five families?

Interesting question.

At the time Michael was in Sicily, his assassination would not have lessened The Corleone's muscle or tangible assets. So on paper, Vito would have had the same means as before.

The problem with that is twofold, both on the psychological realm. First, Vito would have been psychologically beat down by Michael's death. Sonny's was bad enough but losing two of his three sons to the life would likely have been too much to bear. It is easy to see him become disenchanted with the life and step aside, with quite possibly Tessio taking over and the family becoming secondary to Barzini's.

Even if Vito did not initally take this path, the perception of The Corleones would have taken a severe hit. With the efforts to kill their two most prominent sons proving successful, their collective enemies would have the feeling of superiority. Coupled with that would be a desire to finish off their enemy before a desperate act of vengeance could be attempted. This would lead to a swift grand scale attempt to finish off the rest of the family, or beat them down into acquiesence as I described in the preceding paragraph.

Nutshell - while tangible assets would not have been affected, Vito would almost certainly not have been able to overcome the psychological ramifications and that would have led to the demise of the family.

I'm not disagreeing with your excellent analysis, Blond. I want to offer an alternative:

Devastated as Vito would have been by the loss of two sons, he still had a third: Fredo. And he had the strength of character to focus on preserving his family.

What we saw of Fredo up to the point of Sonny's assassination was that he suffered a nervous breakdown after Vito was shot. But, he may have had hidden strengths that were never brought out because he was always under Sonny's and Vito's shadows.

Fredo was in Vegas under the Molinari Family's protection (as we heard Michael tell Moe Green later). What if Vito called a Commission meeting and took the same stance he took in bringing Michael home--he would give in on drugs to guarantee Fredo's safety in returning to NYC. The other Dons would agree because they had little regard for Fredo and were greedy for Vito's police/political protection for the drugs rackets. Then he'd have to concentrate on schooling Fredo and building him up to be his successor. That'd be a big unknown. But, maybe--just maybe--Fredo would rise to the task. Keep in mind that the really stupid Fredo--who took sides with Moe Green against the family; the cuckold and failure in II--was not apparent at the time of Sonny's assassination.

Vito probably would figure he'd have nothing to lose in trying to groom Fredo. Would he be putting Fredo in mortal danger as Don-in-waiting? As against that: what would be Fredo's future if Vito threw in the towel?

I think at that point, Vito would start looking for the door. He'd probably focus on getting as much of the business as he could to Tessio and Clemenza, while getting his real family to a place of safety.

Vito was very clear about what he would do if something happened to Michael. There would've been no peace. And even if there was, the other dons would stay on guard because they knew Vito would never forgive the death of two of his sons. I also think Vito would try to groom Fredo, because he wanted a son to succeed him. The question however is, would Clemenza and Tessio and the rest of the family go along? I think Vito would be able to kill Tattaglia and Barzini, but problems would arise concerning the future leadership of the family.

From what we generally know about the Mafia, a family's line of succession rarely runs through the bloodline. However, in the Trilogy, that bloodline succession is rock solid. I think that B brings up a good point that doesn't get the consideration that it should: what role do the capos play in that succession? Also, what about the other families? Yes, we know of Barzini's machinations. However, what about the Commission's role and the non-violent influence of other families? I just don't see Vito being able to pass the baton to Fredo just because.

Vito was able to pass the baton to Michael--at the time known to the Commission only as the guy whose murder of Sol and Mac caused the Five Families War of 1946--because they got his protection for the drugs trade in return. Why wouldn't they do the same for Fredo, who they would certainly not see as a threat.