What ever the facts about whether any are left alive in Viet Nam, there are still missing...even if "just" their bodies.
As I see it, it's all about "all present OR ACCOUNTED FOR"...leave no man (woman) behind.
When I see that flag, to me, it stands for every soldier that was never found -- alive or dead -- since we became a country (including Viet Nam) and should fly at all times.
The American Flag can stand for all those who returned -- alive or dead, or are buried where we know where they are.
But, these missing are a part of my family. They are the fathers and brothers, mothers and sisters -- sons and daughters -- of my country who gave it all so we are ALL free...but never go their due.