US objective in WW2 was the defeat of the third Reich and Japan while their objective in the Vietnam was to defend ally territory.
The nature of the incomplete war and irregular warfare made the disputation bad.
Step-by-step explanation:
The second world war was in many ways a rather clear cut conflict for the US that was joined after the security of the US base was compromised and they had to make sure it would not happen to their soil again.
They were fighting against an oppressive regime under the support of their people to attack.
The war in Vietnam was defensive and was to defend the territory of the allies.
It was also largely not supported by the people and US did not have much of a freehand.