Excluding the BS wacko enviro argument off the top, I’ll note that China occupied Vietnam for 800+ years beginning about 150 or so B.C. My guess is that China is also the answer to the end of the Khmer and the Angkor Wat group; probably swallowed by China and forced into slave labor in China.
“China” consists of many different groups of people. You can still go to areas where the city next door doesn’t speak the same language as the one you just left. It’s a lot looser configuration of people than we’re led to believe.