It's clear from many different outdoor scenes that the series wasn't filmed in a jungle or in Southeast Asia. Many of the plants shown weren't native to Vietnam in particular , nor Southeast Asia in general.
Many of the Asian actors used were not actually Vietnamese. While Vietnam has (and had at the time of the war) a large ethnic Chinese population, some of the actors used were Koreans, Japanese and even Filipinos.
McMurphy's pilot boyfriend is a Captain (wearing two silver bars on each shoulder) and has Command Pilot wings on his flight suit name tag (pilot's wings with a star surrounded by a wreath). To earn Command Pilot wings you have to be a rated pilot for 15 years and have 3,000 hours of flight time. No pilot in the USAF would be a rated pilot for 15 years and amass 3,000 flying hours and still be a Captain. Besides, assuming he graduated college at age 22 and flight training at 23, and then flew for 15 years he would be 38 years old. Clearly he isn't that old. [See Wikipedia article on "U.S. Air Force aeronautical rating" for rating criteria]