There are some problems with the premise.

First, China wasn't the longest lasting country in human history. "China" was a bunch of states, not always contiguous in time or geography.

Second, It wasn't a cultural monolith, and was just as subject to influences and trends from the outside as other cultures have been. And it too was conquered.

Then, "Vietnam" was conquered. It was a tribute state to Tang. It was conquered by Nanzhao. Korea too was long in a subservient tribute relationship to various "Chinese" dynasties.

These various states didn't exist in a vacuum. Just as "China" was conquered many times by other states/kingdoms/whatever, so too were most of their neighbours.

I'm using quotes here because Vietnam wasn't Vietnam; It was Annam. China wasn't China. It was a whole bunch of stuff. While people regularly talk about Ming as China, or Zhou as China, or Pyu as Burma, these aren't really accurate; They were various different nations with different languages and cultures at different times throughout history, and even though there's a country there today, it doesn't at all mean that we should think it as one continuous country that's been that way for centuries let alone millennia.