Either that or there also wasn't any 'south vietnamese' government during the Vietnam police action, which was at the time heavily supported by the US, but just a conflict between North Vietnam and the US.
Nearly right. The "South Vietnamese" government was an illegal and illegitimate device conjured up by Washington to justify its violent intervention. There was a nation called Vietnam. After international talks, an election was scheduled for Vietnam. Washington decided that the Communists were certain to win the election, so it engineered a "rebellion" by a newly-invented entity called "South Vietnam". Insofar as it ever existed, South Vietnam must have seceded from Vietnam, just as Washington maintains Crimea wasn't allowed to secede from Ukraine.
Incidentally, lots of people are saying that the referendum through which Crimea returned to Russia was illegitimate. It's not often mentioned that, after the dissolution of the USSR in 1990, the people of Ukraine (including Crimea) were not consulted at all about the decision to declare a new nation called "Ukraine". If you consult the history books, you will find that there had NEVER before been a nation state called "Ukraine". It had always been part of Russia, except when it was conquered by the Turks or the Polish-Lithuanian Commonwealth.
And the coup d'etat of 2014 which overthrew President Yanukovych was both violent and illegal - a fact which the present "acting" President Poroshenko has publicly admitted.