In both cases they were/are being supplied with arms from outside sources and are fighting an enemy who is unwilling to actually commit to winning.
I could reference a lot of books and articles about the Vietnam War and the recent PBS series that could clarify things. I would agree with you about not committed to winning if you mean the military and political leaders were totally incompetent in how to fight such a war. Might have helped with the countryside if we had not spent most of our effort on behalf of a corrupt regime, so bad we actually collaborated in assassinating the elected President and then stood by while the generals who took over were equally corrupt and incompetent.
Ike knew something about war when he stopped Admiral Radford, John Foster Dulles, and his brother Allen, who wanted to use a nuclear weapon to somehow save the French at Diên Biên Phu. Do you think Ike was wrong about nixing nukes in jungle warfare? Looks like some in the White House now think they are useful so they plan research on smaller nuclear weapons. (Recently I've read they are for our ships.) I worked with some admirals and have many students with military backgrounds. The Navy is scared of nuclear weapons. They make waves and a naval vessel is a great source of heat so easily targeted. I had lunch with one of the admirals in Washington at a conference. I noted the whole enterprise of Vietnam was strategically wrong because the Vietnamese were actually a great barrier to Chinese expansion in that direction. Then he said, "I commanded a carrier task force off Vietnam during the war and then believed anyone with knowledge of Southeast Asian history would agree with you." Wasn't Trump's second choice for National Security Advsor author of a definitive book on Vietnam castigating the top brass for going along with it?
Dereliction of Duty (1997 book) - Wikipedia
The Navy did want us to use suicide dolphin torpedoes until John Lily found put about it and quit. Maybe that's what Trump has in mind?
My brother was a medic at Danang. Once when they were under siege he was running to the hospital with his buddies when they heard a shell coming, he jumped to one side of the road, they got hit with a phosphorus bomb. Funny how he ran around the back yard the first July 4th after he was discharged. My youngest son was a special forces guy, fortunately he never saw combat. On a visit to his boring Dad he picked up an anthology about Vietnam and remarked, without prompting, "Dad, that war was weird. In my training we are told we have the support of the population. We fought on the wrong side!"
What kind of a foreign policy might we have if Trump had served in our military?
For transparency, I had a lot of 2S deferments, though a new father so not seriously worried about the draft. I did file for conscious objection, initially, but when he found out about it my Dad had a fit so I dropped it. Had MIT a naval ROTC program I would have served. Army ROTC was required, however, and I got an A in drill--probably because I was a good dancer. Normally we didn't think highly of the ROTC instructors—in map reading they tried to help us with tests—can you imagine an MIT mechanical engineer with high school employment as a draftsman having difficulty reading a map? The exception was a really tough captain who taught automotive maintenance. I flunked that test, the only failure in a lifetime of testing. It was a real joke on ourselves in our fraternity; none of us could tune up a car.
Last Edit: Some typos.