In 1957 he returned from the United States and became chief of staff to President Ngo Dinh Diem. Vien had high admiration and respect for Diem and his brother Nhu. During the 1963 overthrow of the Diem regime, Vien, who commanded the airborne brigade, refused to participate in the coup and was imprisoned and condemned to death. Later released, he was returned to command. In the fall of 1965 Vien was appointed chief of staff to the ARVN Joint General Staff and subsequently commanded III Corps. He was later appointed chief of the Joint General Staff, concurrently acting as minister of defense for much of the time.
Vien was a close friend of President Nguyen Van Thieu and their families used to share the same house, but Thieu maintained close control over the Republic of Vietnam armed forces, leaving Vien with little direct influence in military matters. Vien's role was further diminished with the Americans largely directing the conduct of the war. Vien attempted to resign a number of times in protest of American military primacy. He also steadfastly refused to carry out military reforms urged by the United States.
During the 1968 Tet Offensive Vien used all his staff and service personnel, with few exceptions, as combat troops and took personal command of them. Colonels and majors commanded platoons with junior officers filling the ranks as privates. Vien later held that the United States and South Vietnam missed an opportunity to win the war immediately after Tet by not going on the offensive with massive, large-scale attacks. He also complained of not being consulted by the Johnson administration in what he considered its "expedient" policy of "Vietnamization," for which he considered the Vietnamese armed forces neither "psychologically" nor "physically" prepared.
Vien was an enthusiastic advocate of the 1971 ARVN invasion of Laos, Operation LAM SON 719, having in 1965 proposed a "strategy of isolation involving a fortified zone along the 17th parallel running through Laos and an amphibious landing at Vinh."
General Vien appeared in public for the last time on April 27, 1975 at a joint session of the RVN Congress, to which he reported on the deteriorating situation in South Vietnam. He made it clear that the ARVN was no match for the powerful Communist forces that were approaching Saigon. The next day he and his family secretly left Vietnam for the United States, where they settled and he became a citizen. Vien summed up the 1975 defeat by saying that ARVN had "fought well until undercut by events beyond its control."
After his arrival in the United States, Vien worked at the U.S. Army Center of Military History and produced two monographs on his experiences in the war. Ho Dieu Anh and Spencer C. Tucker
Who's Who in Vietnam. Saigon: Vietnam Press Agency, 1967-1968.; Cao Van Vien, and Dong Van Khuyen. Reflections on the Vietnam War. Indochina Monographs. Washington, DC: U.S. Army Center of Military History, 1980.; Cao Van Vien. The Final Collapse. Indochina Monographs. Washington, DC: U.S. Army Center of Military History, 1982.; Kiem Dat. Chien Tranh Viet Nam (The Vietnam War). Glendale, CA: Dai Nam, 1982.; Nguyen Khac Ngu. Nhung Ngay Cuoi Cung Cua Viet Nam Cong Hoa (The Last Days of the Republic of Vietnam). Montreal: Nhom Nghien Cuu Su Dia, 1979.; Post, Ken. Revolution, Socialism and Nationalism in Viet Nam. 5 vols. Brookfield, VT: Darmouth, 1989-1994.
Ho Dieu Anh and Spencer C. Tucker