Didn't use to...but now I believe Vito did know. Remember, prior to that scene Vito and Michael had discussed and planned EVERYTHING that was to happen from the moment Michael took over the business. In this scene after much preparation (including the decision to demote Tom)...they are basically laying the groundwork for what is to come. Everything will eventually fall into placee based on what Michael & Tom present here.

Upon his return home, Michael asked 'What about Sonny; what about Sicily?'. When Vito promised he wouldn't break the peace he made in order to bring Michael home from Italy, he meant to keep to that. But that did not prevent him from discussing with Michael what would happen once he (Vito) was gone.

Another indication is the ride home with Tom after the meeting with the Family Heads after Sonny's death. Vito tells Tom that Sonny could've easily outfought Tattaglia, and he came out of that meeting realizing it was Barzini who was behind everything.

It wouldn't be too hard to figure out that Barzini needed Sonny dead in order for Vito to bend. Even easier to figure out that Sonny is massacred on his way to assist Connie after her hysterical phone call to the mall. Carlo had already expressed unhappiness at not being allowed more involvement in the Family Business. Could Barzini have approached Carlo in order to get Sonny out in the open? Hmmmmmmm.

AppleOnYa