But Vito did have a gigantic part. He states in the novel: "I want nothing to break the peace no matter what the provocation UNTIL we've found a way to bring Michael home." As Tom muses: "All pointed to a day of reckoning in the future." As Mike states in the novel: "The old man planned alot of it." And as far as revenge goes, Tom tells Vito "You say you don't want ... revenge... I don't believe that."
The distinction ya'll make between conspiracy and overt acts doesn't wash.