A comment below suggested another book on Vietnamese history, which I am endeavoring to read in the near future. The comment also brought up issues relating to the ethnogenesis of the Vietnamese people, their relationship to the Yue (or lack thereof) and the Khmer, and also the Han Chinese.
Obviously, I can’t speak to the details of linguistics and area studies history. But I can say a bit about genetics because over the years I’ve assembled a reasonable data set of Asians, both public and private. The 1000 Genomes collected Vietnamese from Ho Chi Minh City in the south. I compared them to a variety of populations using ADMIXTURE with 5 populations.
You can click to enlarge, but I can tell you that the Vietnamese samples vary less than the Cambodian ones, and resemble Dai more than the other populations. The Dai were sampled from southern Yunnan, in China, and historically were much more common in southern China, before their assimilation into the Han (as well as the migration of others to Southeast Asia).
Curiously, I have four non-Chinese samples from Thailand, and they look to be more like the Cambodians. This aligns well with historical and other genetic evidence the Thai identity emerged from the assimilation of Tai migrants into the Austro-Asiatic (Mon and Khmer) substrate.
Aside from a few Vietnamese who seem Chinese, or a few who are likely Khmer or of related peoples, the Vietnamese do seem to have some Khmer ancestry. Or something like that.
Narrowing the populations, and using Indians as an outgroup, I wanted to test the Vietnamese against a few select populations. In the graph to the right you see that they are on the same branch as the Dai, and there is gene flow from the Dia into the Cambodians, and from the Cambodians into the Vietnamese. These results actually suggest that the Cambodians have had more gene flow in than the Vietnamese.
If you check the ADMIXTURE plot though you notice that there is a huge range of variation in the Cambodians in terms of their ancestry. The Mon kingdoms to the west of Cambodia fell to the Tai, but Cambodia itself did not. It probably absorbed a fair amount of Tai ancestry though, even if it retained its cultural distinctiveness and character.
A PCA shows that the Vietnamese are a distinct cluster. Different from both the Dai and South Chinese. Some of the samples in the 1000 Genomes are shifted toward the Cambodians and others toward the Chinese.
Finally, I ran a three population test. Here are some results of interest: