As Jay Leno said, humor is best left to the professionals. And, yes, I have/had family and friends who served in Vietnam. For all of them, the Vietnam was, in fact, an “existential threat”, and for three of them the threat became reality.
It was an existential threat for many men. 58,000+ died. What it was not was an existential threat to this country as evidenced by the lack of any harm to us after we pulled out. It was a side action of the Cold War, the Third World War, the war between two nuclear armed super powers. Afghanistan was never an existential threat for the Soviet Union either. Just another skirmish in the global war then going on between us and them. Do you really think we are “at war” in Afghanistan now? I think we have different definitions of what war is. In my view, if you are not fighting with the intention of winning then you are not in a war.