Tuyến was responsible for a variety of propaganda campaigns against communists, and was prominent in operating the secret Cần Lao party apparatus which maintained the Ngô family's rule.
A short and light man at less than 45 kg,[3] Tuyen hailed from Phát Diêm, Ninh Bình Province, Vietnam, French Indochina.
As a university student, he had protested against the French colonial administration's control over Vietnam's Catholic clergy, [clarification needed] landing him in trouble with the police.
His future mentor Ngô Đình Nhu wanted to travel from Hanoi to a Catholic area near the border with Laos and needed a guide.
A Catholic priest asked Tuyến to lead the way on a bicycle while Nhu followed in a covered cyclo to evade French colonial and Vietminh attention.
[4] In mid-1954, at the time the Geneva Conference had concluded, Tuyến had been working for the anti-communist Vietnamese National Army of the State of Vietnam in an outlying province, only travelling to Hanoi during the weekends.
[3] Tuyến's first task was to disperse the approximately 800,000 northerners who had migrated south during the free travel period in Operation Passage to Freedom before the partition of Vietnam.
Believing they had made a great sacrifice to move, the northerners insisted on settling in or near the overcrowded capital Saigon, which had better urban amenities than regional and rural areas.
Tuyến targeted a clandestine newspaper run by anti-Diệm nationalist intellectuals, by printing counterfeit copies of the magazine with communist propaganda substituted in place of the real content.
[7] Tuyen later supervised the operations of various newspapers that acted as government mouthpieces, including the Cach Mang Quoc Gia (National Revolution).
[11] Tuyen was a key figure in persuading undecided ARVN divisions to support Diệm and put down the 1960 South Vietnamese coup attempt.
Instead, Tuyến responded by dispatching his staff back to their former position they had held before they joined his department, leaving the intelligence bureau in a state of collapse.
With growing displeasure among the populace against the Diệm regime, Tuyến targeted July 15 as the date for a coup, but was unable to recruit the generals required for his plan, since he was too closely associated with Nhu to gain their trust.
It was effectively an exile for Tuyến, and there were rumours [clarification needed] that Madame Nhu's younger brother, Trần Văn Khiêm, was planning to assassinate him.
In April 1975, as South Vietnam collapsed amid a communist onslaught, British intelligence arranged for Tuyến's wife and their three youngest children to leave for Cambridge, where their eldest son was studying.
[19][20] He departed on one of the last helicopters from 22 Gia Long Street and flew out of the besieged city with the help of Phạm Xuân Ẩn,[21] a Time magazine correspondent and - ironically - a communist spy.