North Vietnam at the at the time the game takes place was rule by Văn Lang a Austroasiatic language speaking kingdom in the south which has a largely mythical history but did exist by the games starting Period’ and in the north by Nam Cương Kingdom which spoke a Tai–Kadai language and was part of the Âu Việt an ancient conglomeration of Baiyue tribes living in what is today the mountainous regions of northernmost Vietnam, western Guangdong, and northern Guangxi. Around 258 BC or 257 BC an alliance of Âu Việt tribes lead by Thục Phán invaded Văn Lang and established the Âu Lạc kingdom ruled until 207 BC when a Qin warlord Triệu Đà (pinyin: Zhao Tuo) established his own independent kingdom Nam Viet in the area of north Vietnam, Guangdong and Guangxi. In 111 BC Han China invaded and established territories in the area.
Apart from north Vietnam which was conquered by China during the timeframe of the game (and is the only area of south east Asia that should or could be included in a China dlc) there is very little known about south east Asia for the time the game takes place. The earliest Pyu city-states formed in Burma in the 2nd century BC. The first known states outside of the ones mentioned above is Funan in Cambodia that formed in 1st century AD through Influence from the Indian Sub-continent. Which is to late for the games timeframe.
For some of the places in southeast Asia language cloud be used to try to reconstruct the groups of people that lived in southeast Asia at the time.
South Vietnam was inhabited by the
Sa Huynh culture who were the predecessors of the Cham people, who migrated from Borneo to south Vietnam by 1000 BC.
Thai were not present in the modern region of Thailand at the time the game takes place instead the region would have been inhabited by groups related to the modern Mon and Khmer people. The Thai people migrated from the Chinese province of Guangxi in 8th-10th centuries CE. Though there was the Nam Cương Kingdom that spoke a Tai–Kadai language and tribes in parts of northern Laos that possibly spoke a Tai–Kadai language at that time.
Burma at the time was probably inhabited by the Sino-Tibetan Pyu people along the Irrawaddy River in upper Burma. Though the first of the Pyu city states wouldn’t form for 100 years after the games start. There were probably other Sino-Tibetan groups that inhabited the mountainous regions that divide India and Burma. Lower Burma was inhabited by the Austroasiatic mon people. There’s also the legendary Arakanese Dhanyawadi kingdom for which there’s no Historical evidence for and Definitely did not exist. The Burmans migrated from the Kingdom of Nanzhao in southwest China to Burma from the 8th-10th centuries CE assimilating the Pyu culture and forming the Kingdom of Pagan.
Austroasiatic/Mon-Khmer language family most likely formed along the Mekong river. By the time of the game most people in mainland southeast Asia probably spoke an Austroasiatic language.
There also could have been more Negrito peoples in southeast Asia at that time. Negrito is a broad term for the indigenous peoples that inhabited Southeast Asia before the migrations of peoples from Southern China. Negrito peoples are distantly related to the Papua New Guineans and Australian Aboriginals.
Here a list of information about south east Asia for the iron age.
Sites:
https://www.soas.ac.uk/sbbr/editions/file64275.pdf
http://factsanddetails.com/southeast-asia/Myanmar/sub5_5a/entry-2995.html
https://www.cambridge.org/core/jour.../6D02C584F0EA67B366D3EC5E13D668BA/core-reader
http://bharatkalyan97.blogspot.com/2017/04/hanoi-haifa-maritime-tin-route-meluhha.html
https://www.metmuseum.org/toah/ht/04/sse.html
https://en.wikipedia.org/wiki/Buni_culture
https://en.wikipedia.org/wiki/Igorot_society
https://en.wikipedia.org/wiki/Sa_Huỳnh_culture
https://en.wikipedia.org/wiki/Funan
http://journals.lib.washington.edu/index.php/BIPPA/article/viewFile/9966/9768
https://en.wikipedia.org/wiki/Prehistoric_Thailand
https://en.wikipedia.org/wiki/Văn_Lang
https://en.wikipedia.org/wiki/Âu_Việt
https://en.wikipedia.org/wiki/Plain_of_Jars
https://pia-journal.co.uk/articles/abstract/10.5334/pia.15/