I respectfully disagree, YoTonyB, that it was Tom's fault to schedule the meeting or schedule "only those meetings they could keep and deflect all other proposals that would be unacceptable to the Don." In the novel, it is quite clear that the meeting is scheduled with Tom already knowing that Vito would reject it. Vito once explicitly stated no early in the novel, then later said "How can I make a decision until I know all the details?"

Another reason is also supported in the novel when Sollozzo snatches Tom after the attempted killing of Vito. Puzo states something to the effect of Hagen knew that Sollozzo would only want him to present the peace proposal to Sonny, as a good consigliere was bound to do. In other words, as consigliere, Tom was never in a position of power to make final decisions to Vito. Rather, he was supposed to give Vito his advice/counsel. I would argue that Tom did that - he stated his opinion to both Vito and when Sonny was the Don. In fact, also cited in the novel, when Vito openly asked for Tom's advice regarding the drug proposal, Tom needed to be prodded. Tom was hesitant about telling Vito his true feelings. Vito read Tom's misgivings well and said "Go ahead, Tom. Consigliere's don't always agree with their Dons all of the time."

I fully agree with your assessment that Vito's rejection of the Sollozzo proposal was the true tipping point. It set everything else in motion - Michael finally joining his brother and father in the Family, the attempt on Vito, and the ensuing war.