Pete, I'm not saying Fredo would have been included in any business meetings between Vito and/or Roth. I'm saying that he might have seen them around Vito. In a deleted scene from II, we see Clemenza introducing the young Roth to Vito, who hired him as a truck mechanic. Surely all the Corleone children would have had opportunities to see/meet him. That's why, when Fredo said, "I don't know those guys" [emphasis added], Michael should have smelled a rat.