Air America, the now-famous “front company” airline of the CIA, flew WWII aircraft alongside modern types during the Vietnam War. By now Air America has already been thoroughly written on elsewhere. Less well-known is a similar setup in the same timeframe, also using WWII aircraft: Continental Air Services Incorporated.
(C-46 Commando during WWII.)
(C-46 Commando of CASI at Long Tieng, Laos during 1975.)