"Please get your history right. N.Vietnam started to try to impose their version of Marxist-Leninism on their southern neighbours (a corrupt lot, but that's no excuse) The legitimate government invited the US in as advisers, and it escalated from there. There was no invasion, US soldiers didn't set foot in N. Vietman"
One rather distasteful act by the Americans however was convincing the guys in charge of South Vietnam (names escape me) to allow the Americans to impose their own leaders in exchange for a life in America.
When they agreed America promptly had them shot, washing their hands of the affair of course.
As for the war in Vietnam (and the French didn't exactly have a blameless occupation either what with heads on spikes and the like) wasn't the Gulf of Tonkin incident just an excuse for the Americans to put boots on the ground?
The sad truth is if Ukraine hadn't resisted we wouldn't have given a damn about Putin. Unfortunately for us they didn't. Morality rarely comes into war until it suits the politics.