They started doing business in the black market- regularizing it and bringing business savvy. They also formed consulting companies that government agencies contracted with at far higher expense than just hiring them. The Catholics became the business class and the tech experts.
Also- in the area south of Nha Trang which is a designated Catholic zone the government determined to build a string of plush resorts along the coast to capture foreign tourist money, principally Russian and Australian. They couldn't get enough labor for it.
In 2003 in Cam Đức there were young men sitting around tables drinking at midday- no work. In I believe, 05, the government solved its labor problem and tech expense problems by making the Catholics legal. In 2007 there were no young men hanging out drinking in Cam Đức.
In the meantime the Buddhists, being more rational than, say Moslems, saw how things worked with Catholics and imitated business practices- mainly they began to trust each other in business the way Christians do. Viets are naturally business oriented like Koreans.
Did you visit Cam Ranh? You should see it now. You would not it from what it was in 1970. If you peer around you will recognize the shape of Monkey Mountain but now Cam Ranh is a well laid out fairly prosperous town and the southern anchor of the divided 4 lane coast road that services those resorts all the way up to Nha Trang.
I wish all of them the best.