> We are talking about the vietnam war ... how were soldiers fighting in it retaining the freedoms of the US?

The soldiers that fought the Vietnam war didn't merely retain freedoms, they established practical, though not Constitutional, freedom from conscription.

Well, they were defending the freedoms of South Vietnam, which was a sovereign republic which was being invaded by another country. They were resisting international Communism, which had a goal of establishing Communist governments in every country, to include the United States. So by defending South Vietnam from its invader they were defending the United States.