Outstanding thread.

Sollozzo knew that Tom knew the drug business "was the right thing to do." And whether or not Sonny had his stupid outburst, Vito was not so sure about "this Sollozzo fella," and sent Luca on the fool's errand to the Tatt family.
My guess is that if Sonny had stayed quiet at the meeting,
Barzini would have authorized a shot across the Corleone's bow..namely delivering those dead fishes in Luca's vest.
Because Vito sent Luca to the Tatts for no good reason, he and Tom would realize the killing of Luca was "just business."
Vito was in no position to take the high ground here because he was the one who instigated the espionage. My guess is that the killing of Luca could have allowed Barzini to sweep in
and set up a negotiation with Tom in which it was made clear some kind of compromise had to be made to avoid a full scale war, and at that point a deal would hae been struck. Bottom line? If Sonny had kept his mouth shut whter would not have been a sufficient basis for Barzini to get on board with an attempt on Vito's life.