MC, you (et al) don't make any sense. He's just learned his son and heir has been murdered. He wants to end the war. He wants to insure that he can bring his son safely home from exile. He's tells Tom to arrange a meeting of his rivals so that he can get his kid home. He tells Tom that "this war stops now." So, why in the world would he risk any or all of that not happening by instructing Tom to investigate Sonny's murder? The novel makes it absolutely clear that Vito didn't want anything done about Sonny's murder. In fact, in the novel he tells them not to concern themselves with it, not to committ any acts of vengenece, not to initiate inquiries, not to committ further acts of war, and even to cease protecting their businesses. But you think he said all inquiries. "Hey Tom, look, I want you to find out who killed Sonny, You know, make all inquiries. Yeah, if the other Dons get scared off and don't come to the meeting, don't worry about it. Afterall, Mike likes Sicily and he's met a nice girl over there. And Fredo really likes Vegas. And I don't really care one way or the other about the drug business anymore. So, go ahead and find out all you can." For sure!