There was a very significant difference between the way that soldiers were treated after these two wars.  In general, soldiers and other military personnel returning from World War II were treated very well.  By contrast, soldiers returning from Vietnam were not celebrated and were, at times, treated badly.

After World War II, a number of military figures were given ticker tape parades in New York City.  This reflects the degree to which they were honored and appreciated by the public.  On the governmental level, the GI Bill was created to bring material benefits to the servicemen.

This is in stark contrast to what happened during and after Vietnam.  It is not the case that all returning soldiers were treated badly.  However, they were largely ignored and were certainly not given honors and celebrations.

The reason for this has mostly to do with two things.  First, the US did not win the Vietnam War.  This made people less likely to see the soldiers as heroes.  Second, the Vietnam War was unpopular among many Americans, which is in direct contrast to WWII which was seen as an important and necessary war.