"hearing the Vietnamese "sing Christmas carols" (the Vietnamese are Buddhists and don't celebrate Christmas)"
Vietnamese Catholicism was the reason for the war. Catholic missionaries had great success in the 16th and 17th Centuries. The French invaded to protect Catholics in 1847 and remained there.
The South Vietnamese leadership was predominantly Catholic. Cardinal Spellman financed Dr. Dooley's operations there and strongly lobbied President Kennedy for intervention. Dooley's books stirred American Catholic passions.
American Catholics were strongly stirred up by the clergy about the atrocities committed against Catholics and became the strongest support group for entering the war and remaining there.
The Domino Theory was the stated reason but American Catholic furor was the strongest driving force.
Didn't the Democratic [North] Vietnamese slaughter thousands upon thousands of Vietnamese Christians, a practice that they continue unto this very day?
I worked in a Catholic hospital in my off duty time during my tour in Vietnam. The nuns worked very hard to tend to the sick. They had very little and when we arrived they were so gleeful they were like children. Surgeons operated on crossed eyes, hair lips, and all manner of things that had been left alone. We got in a little trouble for doing it but I am sure there are many adults in Vietnam today that remember Americans fondly. They also ran a school and it seemed to have many children attending.