I think Sol had to kill Vito, no matter what he might have negotiated with Michael at the restaurant. Two reasons:
First: Vito had said no to drugs, and when he recovered, he would still have said no, and would have countermanded any arrangement or accommodation that Michael, Sonny and/or Tom could have made in the interim.
Second: Sol's strategic goal in killing Vito was to gravely weaken the Corleones to get better terms for his deal. As Tom said, "If we lose the old man, we lose our political contacts and half our strength." That would mean that half of the politicos who were in Vito's pocket would be available to be bought by Sol and Tatt. Then he could bargain with Sonny for the remaining half of the Corleones' political strength at reduced rates.
As Michael noted, correctly, the meeting was a stalling tactic so Sol could get more time to take another shot at Vito.
Thing is, how were they gonna get to Vito? As long as Vito was in the house recovering, he was practically invulnerable. They'd have to send a small army of guys to get past Sonny's crew and the other guys guarding the compound after the shooting.
Solozzo I feel was fucked either way you slice it. He was a dead man walking and he knew it.
