Historically, the original Viet/Kinh before been rules by Han-Chinese were considered as Austro-Asiatic which is the same group as Mon-khmer. I red that they originated in Southeas Asia around Red River Delta. The thing that make Viet/Kinh a little bit diffrent from other Mon-Khmer ethnics is they have been rule by Han-Chinese for like 1000 years, So that will explains why they looks diffrent from other Austro-Asiatic (Mon-khmer) ethnics and with heavily influenced of Han-Chinese culture.
By the way the Thai/Dai are originated in South of China and considered as Tai-Kadai ethnic, but move down to Southeast Asia in 13th century. Also the original Tai/Dai genetic are also closely related with in Zhuang/Cantonese in China. Tai/Dai never be invade and rule by Han-Chinese just like Hmong and Zhuang, so culturally their culture aren't close to Han-Chinese culture. By the way genetically they seem to be obviously related to Northeast Asian not any less than Vietnamese. Also Thailand is currently world's bigest Han-Chinese Comunity outside of China, there are 9.5 millions of pure 100% Han-Chinese. Also estimated over 50% of total population are Han-Chinese hybird (30million are Thai-Chinese). So genetically I think Thai are more related to Chinese than Vietnamese .
By the way culturally Vietnamese is obviously more related to Han-Chinese than Thai, but not genetically.
Genetically and culturally is diffrent story and have to be divide apart.
Tai-kadai people were weak and lake in culture, so in the time they move down to Southeast Asia and war with native people of Indo-Chia which is Khmer. Tai-kadai seem to be absorbed into Khmer culture because Khmer civilization on that time were considered as powerful and rich in culture. So this would explains why modern day Thai/Dai in Thailand and Laos are influenced by Khmer culturally.
This is really same case as how Mongolian conquered China but on that time the Mongolian were weak and lake in culture, by the way Han-Chinese were powerful and rich in culture. So those Mongolian obviously absorbed into Han-Chinese culture.