According to Cambodian legend, they are descendants of the prince Kaundinya I of India and the queen of Cambodia Soma.
According to the legend of the Vietnamese people, the Vietnamese are a combination of the two tribes of the mountain fairy and the coastal dragon . In other words, a combination of Thai-Kadai and Austroasiatic/Mon-Khmer. It would like to note that Thai-Kadai are ancestors of Thailand, Laos, Zhuang (in China), Tay Nung (in Vietnam) and Austroasiatic / Mon-Khmer are ancestors of Mon (in Thailand, Laos, Myanmar), Vietic (in Vietnam), Khmer (in Cambodia) and ...
Take a look at the distribution of Thai-Kadai people, you will understand why they are called mountain fairy tribe.
In the old days, there were some Vietnamese scholars who were limited by the thinking of the times. They had no knowledge of linguistics, no knowledge of DNA... So they recorded a beautiful folk tale with quite mythical elements to explain the origin of the Vietnamese people:
Vietnamese legend says that the Dragon's father married the Fairy's mother and gave birth to one hundred eggs. One hundred eggs hatched the first 100 Vietnamese. They are the ancestors of the Vietnamese people.
Once, a Western historian asked a Vietnamese historian humorously: Why do you put this nonsense in your textbooks? Do you really believe that you were born from eggs ?
This legend has been clarified by Vietnamese historians today:
1. Father Dragon and Mother Fairy is about the union between two different tribes.
2. Father Dragon is a coastal tribe that worships crocodile totem Lạc Việt. Mother Fairy is a mountain tribe that worships bird totem Âu Việt.
3. Both crocodiles and birds are egg-laying species. So Vietnamese or Đại Việt people born from eggs are logical thinking (And today, we know for a fact that all women hide their eggs somewhere near their cute belly button).
Westerners believe they are born out of the mud and the morning breath of God, so why does it seem strange to you when the Vietnamese believe in their totem?
And look at Indians and Europeans, they speak languages of the same family Indo-European languages , but they don't really look alike. The difference between Vietnamese and Cambodian is not as far off as Europe and India. I find that they are quite similar.