World War II marked the end of "conventional" warfare, not only from the aspect of weaponry, but politically as well. Vietnam was one of the prominent conflicts the United States engaged in during the 50 year Cold War, which lasted from about 1950 to 2000.
In World War II, there were clearly defined military goals, which supported clearly defined political goals. Not so in Vietnam. Some of the problems were:
1. It was never a declared war. During WWII, the whole of the US got behind the war effort. During Vietnam, the country was split between "hawks" and "doves."
2. Although fought in Southeast Asia, the US was really fighting China, with whom it could not hope to prevail in that area.
3. American soldiers were of a select demographic -- usually lower middle class with no college education, whose average age was 19. Compare that with WWII, where almost all men from all srata of society were involved, and the average age was 26.
4. Media. For the first time in history, television was showing the horrors of the conflict on the nightly news. This was probably the most significant contributing factor to raising people's awareness of what was actually going on.
The term, "severely criticised," suggests that the Vietnam War received more criticism than other wars.
I remember reading about riots in New York City during the Civil War. Civil Disobedience was written in response to the Mexican War. My great grandmother told me about how people criticized the Roosevelt Administration because of the high casualties on Tarawa during World War II.
Some people consider the Gulf of Tonkin Resolution to be a declaration of war. It passed with only one (or two?) dissenting vote. Probably only Lyndon Johnson, Wayne Morse, and Ernest Gruening understood that it was a declaration of war, so much of the criticism must have come from the fact that even people who had authorized the war felt betrayed.
The Media was much more censored during World War II, so there was less criticism during that war, and that might be a reason.
Much of the criticism of Vietnam must have come from Americans who favored the socialist view of government, for example Jane Fonda. During World War II the Soviet Union was America's socialist ally, so the socialists were less critical during that war.