My take, for what it is worth!

• Whilst Sonny's blunder may have been catalyst in bringing Sollozzo and Barzini's [“it was Barzini all along”] plans forward especially with Vito handing Luca on a platter
• It seems to me Sollozzo, Barzini / Tattaglia did not have Plan B There's much they can learn from the Great man Roth!
• Sollozzo and Barzini undoubtedly thought Vito would be dead [five shots and he's still alive] Game on!
• Sollozzo's evil smugness, when Sollozzo told Tom to talk to “Sonny, the caporegimes Tessio, Fat Clemenza and the other two kids” believing Vito was dead and of course Luca was already dead
• My view, the above including Sollozzo's “bad luck for me and bad luck for you [Tom] if you don't make that deal!” and later Sollozzo's meeting with Michael was just a ploy buying time until the murder of Vito Then Vito's Judges and Politicians could be bought over
• I believe it was not about Sollozzo “letting Sonny’s show of greed convince him that Sonny would make a deal even if he killed Vito:” as Sollozzo and Barzini were banking on Vito's death, then canvassing the Judges and Politicians who were in Vito's pocket
• I believe Sollozzo and Barzini would have known “No way was he [Sonny] going to negotiate a deal with Sollozzo” with or without Sonny's greed after Vito's murder
• If Vito had died, I doubt any Judges, Politicians would remain loyal to the Corleones especially with Vito dead and Barzini offering them a lot more money - “all our people are business men, their loyalty is based on that”
• Sure thing “No question that Vito had to go” either way - Yes or No

However when Vito survived his two murder attempts and ”Solozzo made a fatal mistake in underestimating Michael” Bring it on!