The same goes for the Chinese that I have seen born and bred in North America.
Nutrition and weather do change the appearance a bit. But not to the extent of changing the race. A khmer/malay people won't acquire those east asian features even if they have better nutrition.
My aunt will disagree
She's truly white as milk because she eats well, but my mom would also disagree, even thought she have a lebanese look, when she was young she was also very light. I really can't speak for myself, but most malay/khmer have a certain look, they look like darker versions of chinese, well speaking for khmer but they have usually no eye fold. As for me, I recently got tanned, but I can well past for south chinese. You're talking about skin colors or bone structures and phenotypes? Skin color can change drastically, as for phenotypes,
You're choice on what viets looks like, not to metion heavy intermixing in the north by viets and chinese and intermixing between chams in the south.
Northern Vietnamese (just in case you wanna say the southerners are cham or cambodian seeing as 15 mils cambodian and cham affect the looks of 40 mils southern viets.)
cambodians, cambodians are dark and light.
Thais, dark and light, but their is a heavy chinese presence.
Chinese, they look chinese.
Muong, less sinicized viets.