I agree that the Don knew, but did Tom tell Vito exactly where Sonny was and what he was doing? No, but if Vito sent Tom to bring Sonny to the office and Tom returns without him...?? I'm sure the conversation between Vito and Tom unveiled Sonny's whereabouts. Not necessarily in explicit detail (Tom - "Sonny's bangin' Lucy, he'll be here in a minute.") but probably more like, "He'll be here in a minute." I'm sure a "look" passed between Tom and Vito that indicated what Sonny was up to.