I think we probably do know,
the POWs were returned altho a handful stayed on, and I think one was returned 19 y later - and investigated for collaboration. He had to explain what he had been doing for the previous decades
which was kinda different to Dien Bien Phu the catastrophic defeat of the French by the Viet Minh - where very few of those who surrendered came back
The difficulty is that there are the usual anglo conspiracy sites and I think there are 100 000 Vietnam sites run by GIs ( sorry vets ) of varying sanity. Falklands war sites - a handful. Aden another handful
The English just dont get off on Whitehall conspiracy. There was a prog on conspiracy and whether MI5 let off bombs on 7/7 and they couldnt even fill a small bus ....