That's because their leaders either committed suicide or gave up.
Before Japan's surrender they were planning kamikaze operations against the US invaders and believed casualties might be in the millions. They had already launch 2,000 kamikaze operations to that point in the war.
Hirohito ordered his people to submit, and the US allowed him to stay on the throne as a reward. Most of his generals were executed.
If Hirohito himself had been executed, it wouldn't surprise me if there had been a horrific backlash against the occupiers.
I don't know why you mention Vietnam. From their perspective, Vietnam won the war. More importantly, they got the US to do what they wanted: leave them alone.
