Had Michael been killed in Sicily, Vito, weakened by the shooting and devastated by the loss of two sons, still would have had Tess and Clem's loyalties, as well as their formidable regimes. He also would have retained his even more formidable police/political contacts, which the other families needed:
--He would have called the Commission meeting to halt the war.
--He would have agreed to provide political protection for drug trafficking in the East.
--In return, he would have asked for a formal peace, and a return to the pre-war status quo, meaning that he had gambling and unions, Tattaglia had nightclubs and brothels, etc.
--The other Dons would have agreed, since they were getting what they wanted all along. They wouldn't have needed to move against Vito immediately because it'd just rekindle a costly war that they had already won..

I think the other Dons had underestimated Michael as a possible existential threat to them as Don-in-training, which is why they agreed not to interfere with his return to the US. . They would think even less of Fredo. To them, Vito was a beaten man, but they needed his police/political protection. That would give Vito time to think, to plan, to remain the head of the family for several years, as we saw.. He might have attempted to tutor Fredo, in the hope that Fredo would rise to the occasion. More likely, he wouldn't want to expose Fredo to the same dangers that had cost Sonny and Michael their lives. As he approached his death, and with Barzini chiseling Corleone territories, I believe he would have counseled Tess and Clem to make their peace with Barzini and go with him, since he would be gone and Fredo couldn't lead them. They might not go for it, but that would be their problem.
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