Excuse me if this was asked before but I didnt find anything about this in the FAQ section.
I traveled Thailand and just watched "Apocalypse Now" and was wondering: when were the hill tribes and indigenous people of central asia (countries above) "integrated" into the country? (I think the correct term for this is "assimilation" but I am not a native english speaker.)
Thailand, Cambodia, Laos, and Vietnam are in Southeast Asia, not central Asia.
Not all "hill tribes" are indigenous to the regions, and not all indigenous regions are "hill tribes". The Kinh ethnic majority of Vietnam belong to the second wave of people (the Austroasiatic-speaking groups) to people Vietnam some 4000 years ago and is not counted as "hill tribes" by 60s-70s era works of Western writers. Meanwhile, the Hmong, frequently classified as "hill tribes" by 60s-70s era works of Western writers, are relatively recent arrivals compared to the other groups in the region, only moving from China to Southeast Asia at around the 15-17th century.
"Hill tribes" is a slur, at least here in Southeast Asia. Please don't call them that. The more polite term is "ethnic minorities".
"Intergrate" should be the correct term. Assimilate implies that they have lost their ethnic identity. Assimilation does happen, but I think you're asking about the time frame that the ethnic groups were incorporated into the national identity. Furthermore, these ethnic minorities still exist in the modern day and officially recognized by the governments of the countries above, so it's not "assimilation".
It depends on your definition of "indigenous" as to which group is "indigenous" and which group is not. Generally, the ethnic groups in Southeast Asia belong to 5 major linguistic families: Austroasiatic, Austronesian, Tai-Kadai, Sino-Tibetan, and Hmong-Mien. In addition, there are also language isolates, the Negrito groups, and unclassified languages that no one knows about.
Generally, the order of arrival in Southeast Asia for these groups are Negrito -> Austroasiatic (mainland route) ~ Austronesian (maritime route) -> Tai-Kadai -> Sino-Tibetan -> Hmong-Mien -> Sino-Tibetan again (Chinese diaspora from the 17th century onward). However, even among these linguistic groups the time frame of arrival can vary as well, depending on the location. So really, it depends on your definition of "indigenous", which applies more to the Americas and is kinda problematic when applied to Asia.
For northern Vietnam's specific case, the first wave of people, the Hoabinhian, was assimilated by the Proto-Austroasiatic group associated with Phùng Nguyên culture around 2000 BC in northern Vietnam. The Proto-Austroasiatic group in northern Vietnam then split into many branches, one of which became the proto-Vietic group associated with the Đông Sơn culture (1000 BC), which then split into many branches and dispersed all across northern Vietnam and along the Annamite mountain range. One of the descendants of the proto-Vietic group was the Kinh ethnic majority, originally based around Hanoi and later spread to the rest of Vietnam. Then the Tai-Kadai groups and Chinese settlers started to move in from around the 2nd century BC. Vietnam as a national identity (under the name Dai Viet) was officially created in the 10th century, after the independence movement and a civil war (Anarchy of the 12 War Lords event). When the Dai Viet national identity was created, it already included the Viet-Muong groups, the Tai-Kadai groups and the Chinese groups that arrived before the 10th century. After the 10th century mark, the later arrival groups (later Tai-Kadai groups after the 10th century, Chinese groups after the 10th century, Hmong-Mien groups from the 15th century onward, non-Chinese Sino-Tibetan groups from the 15th century onward) continued to be incorporated into the Vietnam national identity at the moment they arrived at northern Vietnam.
For central and southern Vietnam, the Hoabinhian first wave was also assimilated by the second wave Austroasiatic groups. The coastal area of central Vietnam was then assimilated again by the third wave Austronesian group (associated with Sa Huỳnh culture 1000 BC – 200 AD and the Cham political entities). The mountainous regions were mainly inhabited by the non-Viet-Muong Austroasiatic groups and non-Cham Austronesian groups. Meanwhile, southern Vietnam was stuck in a tug of war between the Cham groups and the Khmer (Austroasiatic group and ethnic majority of Cambodia) up until the 17th century. From the 15th century onward to around the 19th century, Dai Viet slowly encroached southward (Nam Tiến southward expansion) and the above groups in central and southern Vietnam were slowly incorporated into the Dai Viet national identity. The process officially finished at around Minh Mạng's reign, in the 19th century just before the French came in.
I dont know enough about the other countries to speak for their cases. I can make some generic summary, but I'm afraid it won't be very accurate.
Resources:
https://science.sciencemag.org/content/361/6397/88
https://www.researchgate.net/publication/281983167_A_layer_of_Dongsonian_vocabulary_in_Vietnamese