Good point F.W. I think you are correct in saying that Sollozzo would have tried to assassinate Vito whether or not Sonny had made his faux pas, however this was in indication of a weakness in the Corleone family that Sollozzzo could have used to get permission from Barzini to take a whack at Vito. Sollozzo also sensed that with Vito out of the way the Corleones would fall in line. When he kidnaps Hagen he not only says Sonny was "hot" for his idea but also that Hagen "knew it was the right thing to do."