The attitudes that Americans have towards Vietnam veterans differs to some degree from the attitudes it has towards veterans of previous wars. (Of course, this is getting to be a very dated question as the number of Korean War and WWII veterans is dwindling rapidly.) The main reason for this is that Vietnam was a very different kind of a war and its veterans had a very different experience both during and after the war.
World War II was very different from Vietnam. So many people served that, in essence, the US was a whole society of veterans in the years after the war. Moreover, the veterans came back as victors in a “good war.” For these reasons, the veterans of WWII did not need as much help as Vietnam veterans did. WWII veterans did not have to do so much in the way of trying to fit in to a society that did not understand them. (The same was generally true of Korean War veterans as the society to which they came back was full of WWII veterans who could generally have understood them.)
The Vietnam veterans had a very different experience. They were treated very poorly when they returned home after participating in an unpopular war. The society to which they returned was not dominated by Vietnam veterans. For these reasons, they suffered much more when they returned.
For this reason, we tend to think we owe them more than we owed previous veterans. We owe them because they were not properly appreciated at the time that they came back from war. Therefore, we feel that we need to make it up to them in a way we do not feel we have to make it up to veterans of earlier wars.