I think that some men are just born different. Vito didn't grow up around the mafia and it's not like he was Henry Hill with the: "As far back as I can remember, I always wanted to be a gangster." For him it was more of a necessity of getting involved in crime, so he had a different mindset than other people.

When it comes to Sonny and Fredo, they had some qualities of Vito, but not the one that Michael picked up (being an "honorable, respectable" man to his family). We can see that with Sonny screaming at the dinner table and discussing business ("Papa never talked about business at table...") etc.

The same way with civilians. Some men cheat, some men don't. I think Vito and Michael were one of those that didn't.

Much like Bobby Bacala & Johnny Sac in Sopranos.

My two cents at least.