Your Vietnam appears to have annexed almost all of Laos and the densely-populated southern provinces of Cambodia, home to an extra 15 million non-Vietnamese combined (roughly 7m Laotians, 7-8m Cambodians). As a result, the Kinh majority (85% in OTL) is reduced to around 75% in TTL, assuming similar population growth patterns to OTL.
This is, of course, a big "if": OTL Indochina's population was considerably lowered by three decades of war and the resulting refugee waves, followed by the Khmer Rouge genocide in Cambodia and the continuing one-child policy in Vietnam. A poor Vietnam that neither suffered a major war nor enjoyed an economic miracle could have a considerably higher population than OTL's 100 million, with a young population pyramid similar to Nigeria. A rich Vietnam, on the other hand, could have had a much earlier demographic transition, cancelling out the population gained through peace and a lack of state-mandated population control - leading to a similar, or even slightly lower population compared to OTL.
The state of ethnic relations will depend on what kind of country this Vietnam is. If it's developing, authoritarian and nationalistic, then political unrest and insurgency is guaranteed; if it's rich, liberal and tolerant, then assimilating the Laotian and Cambodian population would not be
impossible, but still exceedingly difficult. This is because in both culture and language, Laos and Cambodia simply have far more in common between themselves and Thailand than with Vietnam, which is basically an East Asian culture that happens to be geographically Southeast Asian. Quoting my own comment on Vietnamese territorial expansion in another thread:
So instead of Vietnam absorbing all of Laos and most of southern Cambodia, you could have a rump Cambodian state centered around Phnom Penh, perhaps acting as a buffer state between Siam and Vietnam; all of the northwest would still go to Siam, while Vietnam would make do with the
sparsely-populated northeast and just some border territories of the more populous southeast, which are far more easily digestible. As for Laos, much of the country's small population lives near the Thai border; as a result, Siam could easily absorb western Laos, while Vietnam can take over and assimilate the
basically empty east and northeast.