So, why, either in the lead up to the meeting, or during, or after, didn't Barzini attempt once last ditch effort to take out Vito? He clearly drove himself, with seemingly no back up, no protection. Could easily tailed him on the way to the meeting, or had someone in the Corleone operation flip and set Vito up at some point beforehand, or basically did what they did to Sonny on the way back.
This is literally probably the best opportunity Barzini has to finally take Vito out and render the Corleones basically defunct.
So why doesn't he take it?
Because, ironically, he and the other Dons needed Vito alive--to settle the war and to protect the drugs business.
In the first, misguided, scenario, killing Vito would cause the Corleones to lose half their political protection, meaning that the Corleones' cops and politicos would be up for grabs. Then Sol and Tatt would be able to leverage a greatly weakened Sonny to get access to the remaining cops and politicos.
That scenario depended on no war breaking out --Sol convinced himself, Tatt and probably the other Dons of that because Sonny was hot for his deal and Tom knew it was the right thing. But, Vito survived and a long, costly war ensued. With bullets flying, probably most of Vito's police/political allies stuck with him because they didn't know which side to take. By the time of the Commission meeting, the war was at stalemate, business was suffering--and there was still no deal on drugs. With Sonny dead, Fredo useless and Michael an unknown quantity, Vito was the only one left who could negotiate an end to the war and provide the police/political protection needed for them to make millions in drug trafficking.
