Well, it wasn't that the war was outright lost by the US. North Vietnam was bombed to the peace table by Nixon in 1972. The US pulled out and North Vietnam invaded again in 1975, but Congress declined to support South Vietnam. It might be a fine point to make, but the war that started in 1954 did end in 1972.
As far as a change in world order, there is an argument that fighting in places like Korea and Vietnam showed the resolve of western powers to fight Soviet expansion. The Domino Effect idea was that letting one nation fall to the Soviets and doing nothing just prompted others to do the same, so we had to fight. Given how many nations in Asia did fight Communist insurrections with only a few falling to the Communists (namely Cambodia) there may be some validity to the argument. The US certainly was involved in Central and South America in the 70s and 80s propping up various regimes with the eye on keeping them from going like Cuba.
I think the other thing to remember is that places like Vietnam and Afghanistan were proxies during the Cold War. We could fight and support our causes in places like that without actually directly fighting the Soviets (and vice versa).
Regarding your comments in the first paragraph, I agree completely.
America did not LOSE the Vietnam War, it negotiated a peace and left. When the North continued it's aggression, America chose not to fight.
It must be remembered that America signed a treaty with the South Vietnam government, which stated America would support the South completely should the North continue it's aggression. America failed to honor this treaty due to Nixon's Watergate difficulties and Ford's inability to persuade the D Party to honor the treaty.
The D Party, which controlled Congress at the time, refused to support the South and honor America's treaty obligations.
The world knows they can't trust the word of an American politician. When will Americans realize this?
The South Vietnamese government was corrupt and wanted to control the drug trade in the Golden Triangle for profit. Backing them was a huge mistake that was eventually rectified.