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.
