Many explanations have been offered for the case of China and Korea, from humiliation complex, government encouragement, to the widely irresponsible accusation that the mass population was "manipulated" through systematic education and brainwashing. Yeah,
It strikes me that no one seems to mention the "insecurity complex", which IMO perhaps plays the single most important role. The reason Americans are not concerned about another Pearl Harbor is not because Japan is a democracy. It is because US is stronger militarily, and will still be even if Japan re-militarize. It was overlooked by westerner because it never occurred to them that people do realize Japan is a stronger neighbor to the East Asians. It was conveniently ignored by Chinese and Korean because they do not want to admit it.
Historically, China, Korea did not win the war by themselves. US played the most important role in defeating Japan in WWII (This is not to belittle the efforts of Chinese/Korean 'anti-japan' resistance/guerilla pre-1945, which are important in winning WWII. But the war would have lasted a lot lot longer without US and it is unclear if Japan would have been defeated).
So today's concern and may I say 'fear' of japan's re-militarization is REAL in China and Korea. Because they are not sure if they can defeat the invaders. so they are especially sensitive to every single sign of change. Japan's defense expense was larger than that of Korea and China combines until last 2002. And that is because Japan's hands were tied by the 1% GDP cap (as McArthur wrote into its constitution). Japan also has the superior technology and industrial infrastructure ready for a re-militarization, including nuke expertise and plutonium enough to produce 4500 bombs. China, on the other hand, has 300-500 outdated bombs and just a few nuclear power station to supply the plutonium for purification.
This is fear, and insecurity, blind nationalism cannot explain the widespread reaction from Japan's neighbors.
As for vietnam, they know they have a good chance of defending their country again. They did it once. They are not afraid. Plus a number of other reasons (see below).
So it is "insecurity complex", which plays a much larger role than the so-called 'humiliation complex'. Because, Korean economic achievement should take away any reason for Koreans to feel humiliated. China is still far behind in GDP/cap, despite the growth and trend, so this argument cannot apply to China, yet. But this also means China is much weaker when there is a conflict.
Note: Among other explanations for Vietnam's case
1. The life after victory in 1975 was miserable
2. America took many refugees in late 1970s, and these refugees had decent lives abroad, and send money back home, and tell them the life in US
- they also tell that many american people disagreed with the war, and such domestic pressure played important role in ending the war
3. No ‘humiliation complex’ or 'insecurity complex'
4. Recognizing the war was about ideology, not about its people and resources. So the memory of war fades in a few years.
Comparing US in Vietnam 1968-75 with Japan in East Asia in WWII brings about many differences, for example
1. American people treat this part of history with no pretense
2. Atrocity was widely reported in US, but much rarer compared with japan’s. And such cases got treated seriously in US
3. American did not go there to exploit the people or resources