how do you know, then how do people say the southerners are mix cambodian or cham, and not just more sunburned? Thats right, they can't, do you think viets know if they have chinese admixture, i don't know but a 1000 years of domination sounds like a little some some between viets and chinese. I never seen north vietnam in person, but I have seen pictures, and as for pictures between the north vietnamese and the purer muong northerners, i have to say your ancestors may have looked less like viets of today, and one seem to mentioned that mon-khmer people are not austronesian like, the further north the less mixed austronesian mon-khmers are. Most people assume there is no relation between vietnamese and other mon-khmer, but when you see a mon-khmer such as de'ang, palung and so so you'll see unmistaken simmilarities, as you would also see in a khmer person with light skin. And for the epicanthics part, you do know that its not rare among mon-khmers, even khmer who are mix malay have epicanthics 40% of the time.
one of the mon-khmer tribes, what vietnamese could have looked like.
and i'm sure chinese didn't mix negritoes, they never co-existed but there is some australoid traits. One fail to mention that not only the people of southeast asia, but also south china have australoid traits, thats why most southeast asians are just darker version of south chinese. Excluding Viets, they're usually light, tho southerners do get more sun. I don't know what northern tai look like, but the southern thai get more sun, but you can't judge a thai because they're so mix.