I think it was all FFC's doing. He staged that scene to illustrate all of the Corleone power brokers in on a decision just like he staged the post-Vito assassination when all of those power brokers (including Michael) were in Vito's office trying to figure out what to do next.

If I were Vito I would want all of them there so that Sollozzo would understand the Corleone unity. I've never believed that Sonny's gaffe was because he was somehow maneuvering for his own operation. Sonny's question was a practical one and I think that Puzo's and FFC's presentation of it is a hole in the plot. Vito's no was enough reason for Sollozzo to go after Vito.