The Vietnam War was a very different beast to WWII.
Vietnam was not threatening to attack the US but rather the US invaded Vietnam to try and curb it falling under communist rule backed by the USSR. It was essentially a proxy war of opposing ideologies.
The US of course sent thousands of young men to their grave in a foreign country that was alien to them and that they had no concept of how to fight in.
The US lost that war but not before thousands of body bags started returning home and the US position became untenable. Sadly the lessons of Vietnam have still not been learned and it's an ugly stain in the USA's history.
Thankfully, the UK, at least that time stayed out of it.