If you want to run business effectively in one country, you have to understand that country culture, language, geography, and history). Similarly, American were supposed to understand Vietnam in order to fight in this land.
Where did American soldiers learn Vietnamese in the war?
Not many American soldiers took the official Vietnamese classes because they were not based in one place. The military life meant travelling throughout the country to all regions. Just some American military personnel were given some Vietnamese lessons to help them speak and understand some basic Vietnamese. Those courses were usually 6 weeks long. Some senior personnel who were advisors or communication specialists took Vietnamese courses, which were 35 weeks or 47 weeks long. The 35-week course was Saigon dialect, and the 47-week course was Hanoi dialect, and there were Vietnamese professors who provided the instruction. The teachers were rotated every two weeks, so the students would hear various dialects and the differences in male and female speakers. The communications specialists were taught to listen to the radio broadcasts in Vietnamese.
In general, most soldiers and sailors had no language training. They learned a few words and phrases, as they needed them, for their work and off duty time.
What was the technique of the Army’s Defense Language Institute?
Students were exposed more than they could possibly remember. Therefore, what they did remember was embedded in their mind forever.
Even at universities, learners followed a method devised for special troops dropped into the China, Burma and India theaters of the World War II.
Learners drilled with a native speaker under the supervision of an academic linguist who might not speak the language at all. It worked because the Army selected students who were good with puzzles, remembering sounds, and having musical ability.
Great thanks to Pete Young for helping me write this post. He used to be a soldier in Vietnam from 1967 to 1973. You may not know this fact: he still remembers quite a lot of Vietnamese although it's over 40 years since he left Vietnam.