Viên attended the French-run Cap Saint-Jacques Military School, graduating with a commission in the Vietnamese National Army as a second lieutenant in 1949. He rose quickly through the ranks, becoming a battalion commander in 1953 and major in 1954. He attended the Vietnamese National Military Academy as a lieutenant, where he met and became friendly with many of South Vietnam's later military leaders. He twice served in military intelligence (in 1953 and 1954), and twice as a military logistics officer. After the formation of the Army of the Republic of Vietnam (ARVN) in 1955, he was appointed chief of military logistics for the ARVN Joint General Staff. He graduated from the United States Army Command and General Staff College in 1957. By 1960, he had completed parachute training with both the Vietnamese and American military, earned his Vietnamese combat pilot's license, and earned his American combat helicopter pilot's license. Viên was promoted to lieutenant colonel and appointed Chief of Staff of the Special Military Staff in the office of the President of the Republic in 1956. He and his family moved to a modest home in the Cholon neighborhood of Saigon (where he lived until April 1975). He was promoted to colonel in 1960 and named Commander of the Vietnamese Airborne Division in November 1960. This came after Colonel Nguyễn Chánh Thi and Lieutenant Colonel Vương Văn Đông, the two highest-ranking paratroopers led a failed coup attempt against Diem and fled into exile in Cambodia. Based on his experiences, Viên concluded in 1961 that the Viet Cong were no longer acting alone but were being led and reinforced by regular units of the People's Army of Vietnam (PAVN).