It undoubtedly would have looked bad for Sonny "if he'd said yes to the deal, he might have been perceived" that Sonny wanted Vito out, to take over
From the novel (the scene where Vito discusses the upcoming Sol meeting with Sonny and Tom):
The Don puffed on his cigar and said, "Santino, what do you think?"
Hagen knew what Sonny would say. Sonny was chafing at being under the Don's thumb. He wanted a big operation of his own. Something like this would be perfect.
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.
