Korea doesn't compare with Vietnam. That was the first mistake. It's sad that you cannot even see the difference between the two: Vietnam was a French colony: the French pulled out and the US took over their role. That doesn't even remotely compare with Korea.
Secondly, NVA troops only invaded South Vietnam as the US stepped up its military operations. This was deliberate US policy, as the staged Tonkin incident showed. In short, the US waged an undeclared war against North Vietnam, ultimately extending that war to Laos and Cambodja. But I'm sure all this was quite 'honorable'.
As for Dachs, what are Dach's creds compared to K.W.Taylor's that I linked to earlier
http://lrc.cornell.edu/asian/faculty/bios/kwtaylor cv
Taylor served in Vietnam, speaks Vietnamese, has written and lectured on the history of Vietnam in Vietnam.
Argument from authority. Interestingly, you still fail to address any point made. Which, given the first, isn't really surprising.
And finally, we had a choice, support South Vietnam when the NVA invaded as we had promised them when they agreed to the untenable truce conditions, or sit by and watch them collapse under the NVA onslaught.
One was honorable, the other wasn't ... we chose the other.
And lost. Now what does that tell you? South Vietnamese losses were massive, the regime itself decrepit. Since when are military operations governed by what is 'honorable'? Your argument only showed the Nixon administration wasn't honorable at all. That's hardly a surprise now, is it?
Lastly, how does 'doing the honorable thing' relate to 'the Vietnam War could have been won'? (Hint: what they have in common is that they are both myths.)
The basic premise of 'we could have won the Vietnam War' is: the US did not do enough. Considering the fact that North Vietnam became the most heavily bombed country in history, that's somewhat untenable. Despite continual increase of US involvement in Vietnam, South Vietnamese losses kept mounting (ending up being tenfold US losses).
Secondly, the basic premise of Vietnam was to contain (and possibly roll back) Communism in Vietnam. This, however, is a
political guideline, not a military one. De facto the US declared war on North Vietnam, short of invading it. There were sound reasons for the latter, because (as in Korea) China would not have kept idle while a friendly neighbour was being steamrolled by US dominated forces. That would be the link with Korea and a lesson that was well learned. Given this promise, there is some foundation for the idea that the US were fighting 'with one arm tied on their back'. Not a big one, but still. It still doesn't amount to anything close to 'the Viet Nam war could have been won'. That, ultimately, was a mirage which even the Nixon administration realized with its 'peace with honor' slogan. If things would have been left to the military, the war could have escalated beyond control. But thank God, the military don't rule the US - yet. (Another lesson from Korea.)
Destroying the existing government in hopes that a "better" one can take its place is the basic premise of a civil war. The whole "the people will rise up and welcome us as saviors" premise was rooted in the idea that they were having a civil war, but the rebels were incapable of any military action without our "help".
Civil war in Iraq followed US invasion. In essence, the US destroyed a stable regime and got a big mess in return. Now who could have predicted that?