President Ngo Dinh Diem was a Catholic and brother of the traditionalist Catholic Archbishop Thuc. Diem governed according to Catholic principles, even legislating laws more in accordance with Catholic teaching. He was one of the several assassinated by the CIA coup right before the Vietnam War began. Some consider Diem a martyr much as some French consider King Louis XVI a martyr.
The Vietnam War began over a dispute between the government, run by Catholics (some even say it was nepotism), and Buddhists. Diem enacted a law that said a religious group had to petition the government first before being able to fly a flag for a religious holiday. Diem's brother Abp. Thuc celebrated his 25th anniversary of his episcopal consecration civically and with great pomp, but thereafter the government denied the Buddhists the ability to fly their flags on their religious holiday (6 May 1963), and the Buddhists became violent in protest (hence the memorable pictures of violent Buddhist self-imolations / suicides). Thereafter, there was a CIA coup to assassinate Diem (2 Nov. 1963) because, as Johnson later said, Diem was unfit for governing Vietnam. A few million Catholics regrouped into one region of Vietnam.
Diem had a love-hate relationship with the U.S. and France. France was a colonizer, but France's withdrawal from Vietnam left it unstable. The U.S. could've helped with stabilizing it, but it doesn't seem assassinating Diem helped.
At its core, the Vietnam War began as a religious war of Catholics vs. Buddhists.
Diem had an audience with Pope Pius XII, as mentioned in Miller's Misalliance p. 38.
According to this, in 1966
Pope Paul VI addresses 150,000 people in St. Peter’s Square in Rome and calls for an end to the war in Vietnam through negotiations. Although the Pope’s address had no impact on the Johnson administration and its policies in Southeast Asia, his comments were indicative of the mounting antiwar sentiment that was growing both at home and overseas.