Well spotted, jensdad. Your observation made me look at Quy Nhon/Qui Nhon again, which I had previously dismissed, as my search led me to Phu Cat Airport, which doesn't have any hills nearby.
My new research led me to this:
The base is now covered with commercial buildings while the former runway is now Nguyễn Tất Thành road. The city is served commercially by Phu Cat Airport.
https://en.wikipedia.org/wiki/Qui_Nhơn_Airfield
and to the attached 1967 picture. I thought I knew a lot about airfields in Vietnam, but this was a complete surprise. A major airfield completely enveloped by a city.
Great challenge, nvubu.
If correct, I think jensdad deserves to take the helm.
PS The name for the street down the runway, Nguyễn Tất Thành, was one of Hồ Chí Minh's aliases.