Recall that, at the beginning of the meeting, Vito says he wants to "thank Don Barzini for helping me to arrange this meeting." But Barzini isn't acting like Vito's Little Helper. He sits at the head of the table (and is the only guy smoking a cigarette, not a cigar like the "Moustache Petes"), is the first to reply after Vito's speech, and characterizes Vito's refusal to help Sollozzo and Tattaglia as "not the actions of a friend." And, as soon as Vito says he's willing to do anything to make peace, Barzini jumps in: "Then it is agreed: The traffic in drugs will be permitted and Don Corleone will give it protection in the East." Did you hear Vito or anyone else propose that? He showed his true colors, and Vito recognized them.
The only surprising thing here is that Vito didn't see Barzini's hand before the meeting.

As for Luca being killed:
As soon as Vito said no to Sollozzo, he figured he had to kill Vito to get his way. Period. Luca never had a chance because Tattaglia and Sollozzo had planned to kill him as soon as they decided Vito had to go. By killing Luca, they'd get rid of the Corleones' top enforcer--and by sending the dead fish at the appropriate time, they'd administer a body blow to Sonny and the others.