Another nuance on Vito's worst decision:
Vito was sharp enough to realize that Sonny's greedy behavior at the meeting with Sollozzo would embolden Sollozzo to try something ("I'm a little nervous about this Sollozzo fella"). So, why did he tell Luca to "find out what he has under his fingernails"? He already knew, or suspected, that Sollozzo would try something rash.
IMO, Vito should have told Luca to whack Sollozzo, both to get rid of a potential threat, and as a warning to Tattaglia and the other Dons. Since Luca was going to pose as being "not too happy with our family," he might have lured Sollozzo into a meeting with some street-level drug dealer that he knew, and then murdered both to make it look like a drug deal gone bad. Then Tom would have fed the story to his newspaper contacts. It would generate a wave of bad publicity on drugs. The other Dons might have known that Vito was behind it. Good! Vito's message would be: "No drug trafficking as long as I'm the top Don in NYC." He may not have made it last forever, but he could have had his way for a while--perhaps until he died.
Now, you might say, "But the other Dons wouldn't sit still for that. After all, they lined up behind Tattaglia in the Five Families War of 1946." They did--but not because Vito was opposed to drugs, but because McCluskey's murder brought tons of police heat on them, hurting their regular businesses, and the Corleones refused to give up the killer. If Vito had Sollozzo whacked, Tattaglia, a mere "pimp," would have sucked it up. Barzini would have waited for another opportunity.