There were many anti-Communist armed Catholic groups during the 1st and 2nd Vietnam War (commonly known as 1st Indochina War & the Vietnam War). Perhaps the patch belonged to 1 of the 4 largest groups:
In Tonkin (now North Vietnam):
1 & 2 Catholic Dioceses of Phat Diem and Bui Chu under the leadership of Bishop Thaddeus Le Huu Tu and his protege Monsignor Pham Ngoc Chi. Total troops: 7 bataillions. Vietnamese name: Chien Khu Phat Diem va Chien Khu Bui Chu. The troops also received guidance and assistance from Major Pham van Dong, then Commander of the Light Mobile Units and Artillery of Tonkin. This is the same officer who became the commander of the SD 3 Da Chien.
In South Vietnam:
3. Mobile Unit for the Defense of the Christian Communities (French: UMDC - Unites Mobiles de Defense des Chretientes) under the leadership of Jean LeRoy. (before 1954 when SVN was still called Cochinchine)
4. Sea Swallows Special Zone (Dac Khu Hai Yen) formed in 1959 in the southernmost tip of VN by Father Augustine Nguyen Lac Hoa, a Chinese priest from Kwangtung. The group received training from Colonel Dong (same officer mentioned in the above 1 & 2). Colonel Dong (later Major General) was a very talented officer both in the French Army and later the Armed Forces of the RVN. He was the only Vietnamese who commanded French troops and the only General Officer who put American advisors to work. His strength was in organizing and information-seeking (renseignemnet).
Besides the above 4, some of the well-known groups also have special patches and insignias such as Khoi Tu-ve Cong Giao (Group of Catholics Self-Defense) or Cong Giao Cuu Quoc (Catholics National Salvation).
Perhaps the above information will help bring back memory form someone who could provide you the origin of the patch. Good luck.