Well, that's an intriguing one, LW.
In the novel, the Dons at the convention are described as sharing two characteristics: they're good listeners, and their wills could not be subverted except by death, or utmost reasonableness.

If the Dons' convention had been held shortly after Vito's turndown of Sol, he'd have no motivation to cave in, as he did in order to get Michael back from Sicily. But, I'm guessing, if he saw that the other Dons were lined up unanimously with Sol (not just Tatt), and that Tom and Sonny were still in favor of drugs, Vito, "a man of reasonableness," would have yielded. They'd find a face-saving accommodation: Vito would get better terms than Sol offered, with fewer risks, and the other Dons would be able to rent his judges and politicians. I think Vito at that point would have recognized that continued refusal would have resulted in a war, and that everyone would lose. As top guy, he'd have the most to lose.