In the novel, during the meeting between Sollozzo and the Don, the only other people present are Sonny and Hagen.
To met this makes more sense than the film version. I know it has been argued that Vito wanted everybody else to know that his no to the deal was strict. On the other hand, it was a delicate issue. Nobody could foresee if Sollozzo would come up with something else, an alternative plan.
Vito avoided witnesses. Even Clemenza and Tessio received their orders via Hagen.
And as we saw, even Sonny was one person too many. Vito could have known because of Sonny's behavior during the wedding and elsewhere, but perhaps he saw no alternative.
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