But I always wondered why Vito agreed to meet with Sollozzo at all. The novel states that Vito had put off such a meeting until after Connie's wedding. Thus, he anticipated a problem and he already knew what his answer would be.
I think he had at least two reasons for the meeting.
First, he didn't want to gratuitously offend Tattaglia and Sollozzo by acting so high-handed that he refused to meet with Sol. Sure, he knew that his refusal wouldn't please them, but he probably thought that refusing to meet with Sol would be worse.
Second, I think he wanted the meeting so that all the key players in his family would be present to hear him refuse Sol's offer. He knew about the greed that drugs engendered, and he knew that Tom and Sonny were hot for Sol's deal. So, by gathering them together with Sol for his "no," he was telling Sol that there would be no back door to a drug deal in his family.
