Unpopular thoughts alert!
1) Most people agree that the Korean War was necessary because South Korea turned out pretty amazing. But they also dismiss Vietnam war as just another imperialist crime. I just don't think it's right to judge a risky policy entirely based on the result.

2) I didn't find a teaching job after getting my PhD. But it doesn't mean it was irrational for me to pursue a PhD because you never knew. How many people could foresee the loss in Vietnam before it started (the only war where the US got militarily defeated.)

3) If the Soviets hadn't had to spend enormous resources in Vietnam as a result of US involvement, it may be able to install more Communist dictatorships in Asia (Thailand? Malaysia?). I know you guys hate war. But sometimes the alternative of war is not always peace.