I always assumed Vito just knew about Sonny's infidelities. Remember, he had his ways of finding out these things. Then again, Tom was very loyal to Vito, but I don't know if Tom would consider telling him something like this.
TIS
"Mankind must put an end to war before war puts an end to mankind. War will exist until that distant day when the conscientious objector enjoys the same reputation and prestige that the warrior does today." JFK
