This question is incredibly broad, considering that American soldiers were involved in Vietnam, in various capacities, for around twenty years, 1955 to 1975.

However, I would invite you to consider the fact that Vietnam is not, and never was, the Moon. American troops came from America and returned there after their tour(s) of duty, and generally speaking had access to American news media of various types. The growing anti-war protests from the middle-1960s onward were quite well covered in news of the time. While opinions of the war would vary from soldier to soldier, it would beggar belief for them to have been unaware of growing American discontent.