My view is that if Tom knew Sonny was "chafing at being under the Don's thumb," then others would know it, too--Sonny being an unsubtle fellow. That's why, if he went for the deal if Vito died, he'd fall under suspicion--not only in the Family but outside, too.
On the other hand, in front of the family's primary protagonists, Tom told Sonny to make the deal.
"Generosity. That was my first mistake." "Experience must be our only guide; reason may mislead us." "Instagram is Twitter for people who can't read."
