I hate to say it and I have all the respect in the world for people who fought in Vietnam, but our (American) freedoms were not at stake and basically all you had there was a geopolitical game in which two American administrations had overextended themselves by using draftees and huge sums of treasure in something which a demokkkrat congress had officially declared to be unwinnable.
Reagan's big achievement was in discovering that you play geopolitical games with petty cash and a few handsfull of professional soldiers and soldiers of fortune, and not with vast sums of national treasure and hundreds of thousands of draftees.
Moreover I do not see any basic shame or dishonor in the people who like Allen did not feel any particular need to get involved.
Vietnam was part of our containment policy against the spread of communism. Korea was the precursor. We didn't want to permit the North (Communsists) from taking over the South. We were aiding the South and the Soviets and Chinese were aiding the North. Initally, support for the war was strong in the US and in Congress.
If you want to learn the real story of Vietnam, check out this primer written by the veterans of the war. As a Vietnam veteran, I found it to be accurate and illuminating.