I have another theory of why Vito agreed to the meeting even though he knew he was going to say no:

Notice how everyone important in the family was at the meeting? It was Vito's way of telling everyone, not just Sol, that drug-dealing was forbidden. He knew that Tom and Sonny were in favor, and he didn't want them, or anyone else, to be tempted to make deals with Sol or anyone else. All of them heard The Word.