I don't know who admin arrangements for this but WI the US, instead of individual replacement rotated whole units through Vietnam. I'm not talking about divisions, but perhaps battalions or regiments as part of divisions?
I'm thinking along the lines of how Australia did it. A regiment (because the US Army is so much bigger than ours I'll talk in regts instead of btns, just as a baseline) would be formed around its officers, NCOs etc, train as a unit before being deployed to Vietnam as a unit, doing its year long tour (accepting casualty replacements of course but keeping together as a unit) and being and replaced rotated out as a unit.
Australia on a couple of occasions deployed btns from warlike service in Malaysia in Konfrontasi period and the jittery period immediately after it, as well as direct from Australia. So I'm guessing the US could do the same from Europe and Korea as well as from CONUS, without reducing nominal strengths in these trouble spots.
Would this make much of a difference, assuming the higher direction of the war is unchanged?