I know I'm not the one you asked, but if I can add you two cents : Vincenzo didn't act in order to help Michael. He acted in order to save the family's honor.
Yes, if Frankie snitched, the Pentangeli 's reputation would be irremediably lost.

At the end of the trial, Tom (?? I think??) turns to him and says in Sicilian:" The honor of your family is intact."

That would be important for Vincenzo. Essential enough to leave his two-mule town to set his brother straight and then go back. Not in the States to go sightseeing.