The lower Mekong region is a fascinating zone from the perspective of human geography and ethnography. Divided between Cambodia and Vietnam, until the past few centuries it was, in fact, part of the broader Khmer world, and historically part of successive Cambodian polities. Vietnam, as we know it, emerged in the Red River valley far to the north 1,000 years ago as an independent, usually subordinate, state distinct from Imperial China. Heavily Sinicized culturally, the Vietnamese nevertheless retained their ethnic identity.
Vietnamese, like the language of the Cambodians, is Austro-Asiatic. In fact, the whole zone between South Asia and the modern day Vietnam, and south to maritime Southeast Asia, may have been Austro-Asiatic speaking ~4,000 years ago, as upland rice farmers migrated from the hills of southern China, and assimilated indigenous hunter-gatherers.
But the proto-Vietnamese language was eventually strongly shaped by Chinese influence. This includes the emergence of tonogenesis. Genetically, the Vietnamese are also quite distinct, being more shifted toward southern Han Chinese and ethnic Chinese minorities such as Dai. My personal assumption is that this is due to the repeated waves migration out of southern China over the past few thousand years, first by Yue ethnic minorities, and later by Han Chinese proper. Many of these individuals were culturally assimilated as Vietnamese, but they clearly left both their biological and cultural distinctiveness in what was originally an Austro-Asiatic population likely quite similar to the Khmer.
As I have posted elsewhere it is also clear to me that Cambodians have Indian ancestry. Because unlike Malaysia Cambodia has not had any recent migration of South Asians due to colonialism, the most parsimonious explanation is that the legends and myths of Indian migration during the Funan period are broadly correct. There is no other reason for fractions of R1a1a among Cambodian males north of 5%. Depending on how you estimate it, probably about ~10% of the ancestry of modern Cambodians is South Asian (the Indian fraction is easier to calculate because it is so different from the East Asian base).
This is present in a few Vietnamese (Kinh) samples I have seen, but it is at a lower frequency. The reason for this Indian ancestry is that southern Vietnam became Vietnamese only in the last 500 years, and more intensively only in the last 200 years. The Vietnamese with Indian ancestry are almost certainly people who are from the southern part of the country with Khmer, or Cham, heritage.
Viet Nam: A History from Earliest Times to the Present is divided into three broad periods. The first is the development of the Vietnamese people as a synthesis of external elements from the north, and the Austro-Asiatic “sons of the soil.” Roughly from the Trung sisters down to the emergence of an independent Vietnamese state in the decades before 1000 AD. This is a narrative of perseverance. Unlike the Yue people of Guangdong and Fujian (and parts further north), the Vietnamese maintained their ethnic identity through long periods of Chinese rule. Transformed and reshaped by the Chinese rule, they emerged from it inflecting Sinic cultural elements within their own traditions.
The second phase is one of conquest. To some extent to an American who is used to seeing the Vietnamese as being catspaws in 20th-century geopolitics, it is painful to read about the drive south of the Vietnamese, and their extermination and assimilation of the earlier peoples and polities. Though they did not use a word such as “Manifest Destiny,” with hindsight it was clear that the Vietnamese were going to push along the coast southward until someone stopped them by force. As it happened, the rise of Vietnam coincided with the decline of Cambodia.
In the 18th and early 19th centuries, Vietnam and Siam (what became Thailand) fought over Cambodia in a manner analogous to occurred with Poland in the same period. The Vietnamese rule of Cambodia, especially in the first half of the 19th century, was concurrent with a drive toward more punctilious Confucianization of Vietnamese society along with a drive to forcing Buddhism into the private domain. This Confucianization entailed reinforcement of patriarchal rules, as well as attention to matters of uniform dress. The Vietnamese monarchy was attempting to create a Confucian society ruled by virtuous bureaucrats, overseeing a populace aware of and cognizant of the proper civilized forms.
Though never as extreme as Korea, Vietnamese Confucianism during this period was probably more pervasive than it ever became in Japan (where formal Confucianism tended to be the purview of the samurai class during the Tokugawa age). As part and parcel of civilizing Cambodia, making it Vietnamese, the conquerors attempted to do with the Khmer what they had done to their own people. Diminish the role and prominence of institutional religion, in this case, Theravada Buddhism, and educate the populace so that they could begin to produce their own virtuous bureaucrats.
One of the most interesting and curious aspects of the Vietnamese rule of the Cambodians is that the comments by the ruler of Vietnam and his subordinates clearly show some deep lack of the understanding of the distinctive nature Khmer culture as opposed to Vietnamese, in particular, northern Vietnamese, culture. They complain that though the Khmer maintain outward forms of proper decorum, they seem not to internalize the forms in a manner that would indicate they are sincerely civilized. The Vietnamese ruler marvels that the Cambodians have 1,200 years of history, but lack precise dates on their origins, and have vague dynastic periods (this is, to be frank, a very Indian feature). Additionally, the Khmer seemed obstinately attached to their Theravada Buddhist religion. When they rebelled against their Vietnamese overlords with the aid of Siamese invaders they declared that they did so to defend the Three Jewels of Buddhism. As is common in China, Vietnamese Buddhist sects periodically rebelled. But these rebellions were sectarian. In Cambodia Buddhism was not a sect, to be a Cambodia was to be a Theravada Buddhist.
In frustration, the Vietnamese ruler declared that “moral suasion” simply does not work with the Khmers! Though his regime was brutal, he was ultimately a Confucian who assumed exhortation would win out in the end.
Though the Vietnamese were aware of the cultural differences between themselves and the Khmer, they were not prepared for the task of swallowing a whole civilization distinct from their own.
This brings to mind comments of Victor Liberman, a scholar of mainland Southeast Asia, that Vietnamese Sinic Confucian statecraft was qualitatively different from the “solar polities” to its west. In his book Strange Parallels Southeast Asia in a Global Context, he outlines what he believes to be the features of these societies which allowed them to emerge in the early modern period with nation-states in a manner recognizable to Europeans. Over most of Southeast Asia Indian high culture spread in the period before 1000 AD (in fact, it was dominant in the southern two-thirds of modern Vietnam before 1500 AD). This meant the emergence of relatively politically loose societies around the charismatic figure of a monarch whose legitimacy was fundamentally religious and metaphysical. Southeast Asian kings aspired to be cakravartin. The turners of the wheel of history.
In contrast through steps and starts the Vietnamese developed a society which was in many ways a miniature shadow of that of China to the north. Instead of a divinely sanctioned monarchy, Vietnam produced subordinate kings to the emperor of China or in some cases a ruler who declared he was an emperor himself. Their rule was sanctioned not by gods or priests, but impersonal Heaven and its mandate.
Whereas other Southeast Asian monarchs had court brahmins, bhikkhus, and later in the Malayan world ulema, the Vietnamese monarchs often put away the Buddhist monks and priests and hid any religious devotion from public view. On the Chinese model, the Vietnamese drove religion away as a helpmate, and subordinated religious impulse as ancillary to state functions and transformed it primarily to something that was a matter of popular enthusiasm and private devotion. Like the Chinese, the Vietnamese polity aimed to recruit and produce a large and broad class of virtuous administrators, many drawn from the agricultural populace itself to main social order and proper state function.
Liberman observes that the Chinese model necessarily requires greater coordination, concentration, and mobilization. Additionally, there naturally develops a cultural chasm between the simple peasant, and the educated bureaucrat, in such a society. In contrast in solar polities, the king and high nobility may be distant from the people as symbols, but the vast mass of peasants and clerics interact and engage on a popular level. Religious truths and ideals often can propagate on a dimension closer to the masses than the culture of the Confucian literati. While efficient and constitutive mobilization of the resources of solar polities is low at any given time, mass enthusiasm may be easy to trigger in punctuated bursts of activity around charismatic figures and exigent circumstances.