Why did America Denounce Returning Vietnam Veterans?

You are close to the answer. Vietnam was a very questionable war for us to be fighting in the first place. When the public figured this out, they wanted the USA out. The USA did not win this war, so those fighting seemed less heroic. But the biggest reasons were that it was the first real media war, and Americans did not like to see the war every night, on their TVs at dinner time. People saw the dead and wounded, the shots being fired, the horrors of war. There was more media there so more of the bad stuff was reported, just like Iraq today. It is not as if they were booed, it is just that they did not receive a hero's welcome like after World War II. WWII was romanticized to a certain degree, as a quick check of the films at the time will show. The whole country was involved in sacrificing because so many people were involved in the war effort both here and overseas. Vietnam, well, that also came on top of the civil rights movement and three significant political inations. It was a unique time, and it was potent mix that unfortunately caused Vietnam vets from getting a more positive recognition.