I think the viewer has to at least be open to the possibility that no way, no how, here or ever, would Vito Corleone have participated in the drug business, and thus Barzini was well aware of that fact IMO while Sonny and Tom should have also been privy. I think Vito so anxiously awaited Michael's arrival at the wedding because he knew subconsciously he might need him in the impending war that would follow after his refusal of Sollozo and Barzini's deal
Atleast taht's the way I see it.
That's certainly possible, maybe likely. But that begs the question - if that's the case, why would Vito even bother to consult Tom and Sonny regarding their opinions prior to the meeting? Was it merely a test to evaluate their capacity to reason and persuade? I see no other rationale.
