To come at this from a very basic point of view, I would say the lesson is simple: we can't win 'em all. This war brought America down a few pegs (in our own eyes!) on quite a few levels. I'll simply discuss to three of them. First, it damaged our pride in that we learned the United States of America could actually lose to a force (North Vietnam) we considered to be evil. Second, it showed us that our leader (the president of The United States) could be capable of corruption where military action is concerned. Third, it exposed the effect on the war effort of the media being more involved.
One lesson from the Vietnam War is that government perpetrated deception adversely affects the safety of American citizens and our republican democracy while also endangering people and nations in other parts of the world. Case in point, the cloak and dagger escalation of war that preceded the Tet Offensive, while Washington was declaring deescalation, put our citizens on active duty at heightened risk, sabotaged trust in our government, and caused manifold suffering to other peoples--if the war legitimately needed escalation, Washington had the duty, obligation and moral responsibility to be forthcoming about it.
As a former Army Wife, and as a current Army employee, the biggest lesson that I hope people learn is to treat war veterans with respect and not the way they were treated once all Vietnam vets came back. I can imagine how horrible those poor kids- because they were kids!- felt coming back from a war that they did not understand only to have people spit in their faces, and the government forget about them.
I am very pro-military but, even if I weren't, it enrages me to think that someone is capable of disrespecting a soldier who has signed up, in the interest of our freedom, to do a job that I would never dare to do. I hope we remember Viet Nam and never repeat the horrors of the Viet Nam veterans ever again.
I'm reminded of a line from the film The Princess Bride. According to Vizzini,
"Ha ha! You fool! You fell victim to one of the classic blunders--the most famous of which is "never get involved in a land war in Asia."
Sadly, our leaders have not followed Vizzini's advice, adding Iraq and Afghanistan to our foolish blunders following the debacle in Vietnam.
Insurgencies are difficult to defeat, especially if they have strong sources of financial and military foreign aid, and especially if they are nationalist movements. Insurgencies take a long time to control, a lesson that would have served us well in Afghanistan or Iraq, and long term occupation is cripplingly expensive. Lastly, and this lesson we seem to have taken more to heart, the generation of veterans returning home with emotional and mental health issues need a support system for recovery. So far, we seem to be investing in, and paying attention to, that reality.
Perhaps the lesson here is to mind our own business? It seems that wherever our politicians tread the problem becomes bigger than it was before our meddling. Thomas Jefferson and I are among those who believe in a bit more isolationist policy. Of course, the world is getting "smaller" by the minute with our mixed economies, technology, and interdependent workings through government and business. It will be much more difficult to live and let live.
I think the meaning for us is clear: hold those who would send our young men to die in foreign lands to the HIGHEST STANDARD of necessity, and then watch them every step of the way. In fact, I think we should ask the people who send our soldiers into battle to LEAD them at the front.
I confess to this because I was a young college student during the War, and was taken in with the domino theory. Today we have soldiers fighing 2 or 3 (or none, by definition) wars which do not seem to have an exit strategy and which is/are justified, in part, by the idea that it's better to fight them there than here. Whenever I am presented with 2 alternatives, I wonder why only two ... there are almost always other alternatives, sometimes much better ones.
So I would say, be skeptical about entering ANY war and hold our elective officials to the highest standard of proof (in everything they do). As said above, some may disagree with this, but it's my meditation on my experience a long time ago ....
Another lesson that some might see in the Vietnam War is that politicians should be forced to enunciate clear goals for wars and clear methods for knowing when a war has been won and can, therefore, be ended. In Vietnam, our goals were not really all that clear and it was hard to determine what victory would look like. This led to something of a quagmire where we floundered around without a clear goal or plan for achieving that goal.
There is a double irony to U.S. involvement in Vietnam. First, the U.S. under its policy of containment was determined to stop communism in its tracks, and hopefully bring it to an end. Not only did containment not work in Vietnam (or anywhere else in Southeast Asia), but communism as an economic and governmental system collapsed under its own weight. No action by the United States contributed to its collapse. A second irony is that the people of Vietnam were doing nothing more than that which the people of the United States had done in 1776; demanding freedom from a government which they felt did not protect their interests. Yet the U.S. was so determined to stop communism, it did not consider the rights of the people of Vietnam. In essence, we wished to impose our own values on a nation which did not share them. Double standard might be a better phrase than irony.
There are many who may disagree with my assessment; however this is the lesson of Vietnam which I see.