With my limited European understanding of Vietnam's revolution and the wars, I have understood the Catholic church played a role in opposing communism and supporting the Southern regime. I have lately befriended some immigrant people in my country who come from the South Vietnamese community, are all Catholic, and tend to be very conservative and opposed the current (North based, and to what extent still socialist?) regime. They claim to be an example of the majority of Vietnamese people, however statistically Catholicism seems to be a minority religion in the country.
I would appreciate especially an internal view to this: To what extent did the church side with the South in the Vietnam war? Has it been more influential in the past, and diminished since? Are the official figures of religiosity in Vietnam accurate? Has there been a cultural shift concerning religious values in Vietnam since the country became socialist, or are they incorporated in the state's agenda and actually the norm?
Your friend lied. Catholics have never been the majority in Vietnam, not even in southern Vietnam during the war era, not even in Saigon itself.
Vietnam is traditionally Buddhist-Taoist-Confucian, and it stays Buddhist-Taoist-Confucian. Christianity is not a native religion to Vietnam and was only introduced during the French colonial period. During the French colonial period, Christianity never gained fraction outside of the elites and middle managers who worked for French. In the modern day, Vietnam is very obviously still majority Buddhist-Taoist-Confucian. This majority is usually registered as atheist/folk religions in surveys. You only need to visit Vietnam to see that pagodas and temples are all over the places in Vietnam, both north and south. Vietnam is home to both the oldest Buddhist pagoda and the largest Buddhist pagoda in Southeast Asia. The CIA world fact book also has statistics from the 2009, last I check, if you want to see stats from a foreign organization. Even in the CIA statistics, you can see that Christianity of all kinds stay a very small minority.
A major event of the war was the Buddhist crisis, as documented and analyzed in the Pentagon Paper. The elite of the RVN government, the Ngô Đình family, were part of the Catholic minority ruling over the Buddist majority in southern Vietnam. Ngô Đình Diệm allowed his brothers and sister in law a lot of power in the government, promoting his brother into the position of Archbishop. The CIA record showed that his family abused their position to power to integrate the church with the state and attempted to create a Catholic state by suppressing Buddhists in the country. Ngô Đình Nhu was recorded to have raided and conducted massacres in Buddhist temples against the US' advice. This eventually led to the Buddhist crisis, where monks in southern Vietnam immolated themselves protested the Catholic elites. The picture of Thích Quảng Đức immolating himself made its way to American media and fueled the anti-war sentiment in the US, eventually leading to the CIA concluding that the Ngô Đình family had gotten out of their control. Later on, because of a myriad of reasons, among which was the Buddhist crisis, the CIA finally decided to depose the Ngô Đình family and replaced them with the RVN military junta. Trần Lệ Xuân, Ngô Đình Nhu's wife and one of the people who led the Buddhist massacres, was the only surviving member of the Ngô Đình family in the US as an exile.
There is a cultural shift in attitude toward religion after 1975 in that it returned to what it had been before the French colonial era. The Catholic elite was deposed. Church power and state are separated again. Vietnam returns to being a secular country, officially atheist, unofficially Buddhist-Taoist-Confucian.
Christian values <> Religious values <> National values. Buddhism, Taoism, and Confucianism have been an integral part of the Vietnamese society for a good 2000 years. Christianity was only introduced to Vietnam at most 300 years ago. Vietnam has its own historical values, both cultural and religious, which are in no way defined by the modern introduction of a foreign religion.