I think being the "home team" makes swallowing those casualties easier (as easy as they can be, anyways); it's easy to perceive the situation as a fight for your life.

Obviously, there were other things going on in Vietnam (and Afghanistan and the larger War on Terror) to keep them fighting but it's much easier to muster up the manpower when a war seems existential because it's happening in your neighborhood.

As opposed to democratic countries like the US or UK which would just lay down their arms after a few tens of thousands of their soldiers were killed in the event of a foreign military invasion on their territory?