After getting what they wanted all along, why chisel Corleone territories?

That's why Vito said no to Sol initially- that his "friends in politics wouldn't be friendly very long if they knew my business was drugs." The novel provides more detail: Sol said at the meeting that he'd use operatives with clean records "so it will be logical for judges to give light sentences." And, after the Commission meeting, Vito told Tom that if the families used "real snowbirds" (meaning guys with drugs arrests) "we'll throw up our hands and say there's nothing we can do" (or some words to that effect). IMO, both Sol and Vito were deluding themselves about drug operatives with "clean records" --greed trumps restraint and common sense every time

Tom: When I meet with the Tattaglia people, should I insist that all his drug middlemen have clean records?
Vito: Mention it -- don't insist Barzini is a man who'll know that without being told

Perhaps he tried to have Michael killed to prevent Michael from eventually avenging Sonny's murder. Then you might ask, why did Barzini agree not to interfere with Michael's return to the US? Because he and the other Dons were greedy for the drugs profits Vito's political protection would open up. They deluded themselves into believing that Michael--who had already shown his stuff by killing Sol and Mac--wouldn't be a threat to them. Greed trumps caution every time

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My only debating point - if I have not misunderstood - it didn't seem Vito was greedy for the narcotics money