I'm guessing that it was Mob protocol. As Lana said, Vito probably decided against the drug deal long before he met with Sol. But, Vito was known as "a man of reasonableness," which helped him in his diplomacy with other Dons during peacetime. Refusing to see Sol, who was in effect sponsored by the Tattaglias, would be unnecessarily rude, and would cause bad feelings--or worse. Vito probably kept putting off the meeting with Sol, which was why Tom pressed him for a date.
BTW: There was more to Vito's opposition to drugs than the threat to his political base. In the novel, he tells Tom: "What he [Sol] will propose is an infamia."
TB, you're probably right that Vito's consent to the Sollozzo meeting was probably protocol and a courtesy to the Tattaglias.
Also BTW, in the novel Vito notes to Tom that, previously, Sollozzo made a living from prostitution which Vito frowned on ("just as the Tattaglias do now").
