I don't know, maybe I'm just retarded. It just seems that tactics have changed since World War II, but I can't really express how. And it also seems to me that those changes have been for the worse.
And again, I may be disastrously wrong, because I really have no idea what I am talking about. This is just my own personal opinion that I came up with. However, because of this thread, I may change that opinion.
They did change, no doubt about that. But that's just because of the agenda and actual objectives of the war in question. The US's objectives and goals in Vietnam were radically different then those of World War 2. World War 2 was a fairly straight forward affair with very little ground breaking in terms of objectives and goals, but Vietnam was relatively virgin ground for the US military and none of it's strengths really counted in the way they wanted to.
For instance, look at Operation Iraqi Freedom. Now, the invasion can be correlated (Note to all military history buffs like me, I know I'm doing a bit of a stretch in comparing the goals, objectives, and tactics of Iraq with WW 2, but bear with me) to traditional WW 2 era battle. Large, mobile forces engaging eachother in set-piece battle. The tactics of which were nearly identical with World War 2 era tactics and the same principals and knowledge which would have been forefront in the US forces in Vietnam War. As shown, the US military is VERY good at these tactics and absolutely crushed the enemy in a manner of weeks. We never abandoned the lessons and tactics of WW 2.
You see the problem with your view? Vietnam was an entirely different situation and we really couldn't use our well known tactics to our stength, so we tried new ones without abandoning the lessons learned from WW 2. We couldn't amass a large body of troops and tanks and roll on to NV for obvious reasons. Indeed, it's a fairly well known fact that the US was close to victory many times in Vietnam and nearly had to have the stomach and political will to end it in our favor. However, due to some faulty military thinking (war of attrition) and being hog-tied by politics - we didn't and we lost.
There was a change in Vietnam, but it was merely the US military trying to adapt to the situation, which, in may ways, it did quite well considering the circumstances. It was only through a combination of bad politics, lack of political will, and a few bad military thinkers that we lost. To this day, me and a lot of other military buffs are still baffled at how the US lost the war, when they had been close so many times. Which makes the fact we left behind so many of our POWs behind all the more sad.