He then went to France to study at the Catholic Institute of Paris, where he earned a master's degree in dogmatic theology in 1970.

Following the reunification of Vietnam under the Communist government of Hanoi, on 7 September 1975, Pope Paul VI appointed him Coadjutor Archbishop of Huế and assigned him the titular see of Tipasa in Mauretania.

He assisted Archbishop Philippe Nguyễn Kim Điền for eight years during the difficult post-war period.

In November of the same year, he obtained permission from the government to reopen the seminary of Huế, and in 1995, he was elected president of the Committee for the Clergy and Seminarians of the Vietnamese Bishops' Conference, re-elected for a second term in the triennium 1998–2001.

Unable to travel to Rome, he received the pallium on 14 August of the same year from Cardinal Paul Joseph Phạm Đình Tụng.