That's an oversimplification of an already complex history of Korea[1], which is why I said equating Vietnam to Ukraine is a disservice, which offers a clearer contrast.
1. The whole Korean peninsula was a colony of Japan, and was divided between the US and Russia after WWII. Both sets of governments claimed to be the the legitimate authority over the entire region. It ended up being a proxy war - so not comparable to Ukraine for a different reason, unless one thinks the war in Ukraine is a proxy war - which completely takes away the agency and sovereignty of the people of Ukraine, and will be provable via a natural experiment over the next days as the US has stopped its involvement, the the war is continuing.
1. The whole Korean peninsula was a colony of Japan, and was divided between the US and Russia after WWII. Both sets of governments claimed to be the the legitimate authority over the entire region. It ended up being a proxy war - so not comparable to Ukraine for a different reason, unless one thinks the war in Ukraine is a proxy war - which completely takes away the agency and sovereignty of the people of Ukraine, and will be provable via a natural experiment over the next days as the US has stopped its involvement, the the war is continuing.