Your finer points do nothing to dispel my argument, in fact, they further make my point.
I'm not blindly accepting anything; the 'popular narrative' is in fact something like 'US involvement in Vietnam is Imperialism'.
US involvement in Vietnam was not an act of Imperialism by any historical standard and it was ballpark consistent with actions in WW1, WW2, and Korea.
The US did a reasonable job of helping to rebuild Japan, a nation that had marauded the region, murdered millions and attacked America (does an Imperial power help their enemies rebuild, institute democracy and leave them better off?), and this included some things like 'getting women the right to vote'. It's ridiculous to think that the occupying Americans would 'right all the historical wrongs' of a very foreign nation levelled to the ground. 'Wrongs' which by the way you've characterized politically (i.e. 'left-wing' vs. 'right-wing') and therefore difficult to consider in a moral context anyhow.
Ironically, were the American occupiers to have intervened further, armchair historians would find yet another excuse to lament it as yet another form of 'Imperialism' or 'cultural imposition of western values' etc..
And your comments about South Korea ... yes, backing an authoritarian leader in the aftermath of devastating war in the mid-20th century is a rational and practical choice. It's not to say that all American choices are perfect, but actual unfolding of history for most S. Koreans went reasonably well by any standard.
Thanks to American involvement in South Korea, Japan, and Taiwan they are all lively, healthy, fruitful, democratic nations today.
Were the Americans to have not walked away from their victory over the VC and North Vietnamese forces, and left a contingent of forces remaining South Vietnam and continue the threat of total annihilation of Northern forces ... South Vietnam would be like South Korea - a beacon of civility and progress to the region.
Sadly, the Americans withdrew, and so I got to grow up with Vietnamese refugee friends in my neighbourhood, who incidentally, all became Dentists. Literally all of them.