There's a Southeast Asia specialist working with Firaxis since early 2020 posting in this thread. One whose focus is the Tai-speaking regions of it (Laos, Thailand, plus all the historical inbetweens)
I wouldn't make too much of it. Academics specialize, game companies seek generalists. Whoever they hired would have a specialty of one sort or another. I could just as easily be a Europeanist. And all of my work at Firaxis involves looking up and reading about things that I have not written about nor studied in grad school. Academia isn't about knowing things; it's about knowing how to find out about things.
About Vietnam being a "Chinese" state - that is in a sense a political argument. Vietnam and Korea were at one point vassals of China or Chinese generals, but have been independent for (nearly) a thousand years. If we're going to say that Vietnam is a Chinese state, we'd better be ready to say that England is an Italian state (b/c Romans), or a French one (b/c Normans), and nobody would be happy with that characterization. The two countries have moved on since then and, in fact, been in a near-constant state of (at least cold) war since. Any claim that Vietnam "is Chinese" is going to be an extremely politically provocative and unfounded one - Ho Chi Minh would have disagreed, as well as nearly every Vietnamese emperor, and on back to the famous sisters that everyone is talking about here. Unlike the dynasties that are considered a part of Chinese history, Vietnamese emperors never made a claim upon the Chinese throne (as far as I know; in this I acknowledge I might miss an ambitious emperor or two).
About Vietnam being a "Sinitic" state - that's clearly the case. Look at a map of Southeast Asia - look at those mountains that make up the eastern spur of the Himalayan massif. Those are a huge barrier, meaning that Vietnam adopted a lot of what you say - Confucianism (and the notion of imperial exams), Mahayana (East Asian) Buddhism, etc. While Vietnamese used classical Chinese, in the 1400s they developed their own, Chinese-derived script, nom. So, again the parallels with Korea or Japan, where Chinese-influenced (but not Chinese) people improvised upon a classical Chinese basis. And also like those two places, the language is in its roots unrelated to a Chinese dialect (Vietnamese is closer to Khmer, although it is highly tonal whereas Khmer is not).
About Siam and Burma. Clearly Vietnam, Siam, Burma, Navajo/Dine, Salish, Assyria, Babylon, Cherokee, Iroqouis/Haudenosaunee, Ashanti, Austria, (catches breath), and so many other speculative civs here deserve to be in the game. But we've only got so much time and resources - and there's no way in
narok that I'm going to drop any hints, or even allow my own area of expertise to
be a hint. It's not.
If Bangkok ends up a city-state and in your game becomes a vassal of Dai Viet (!) or Burma (!!) or France (!!!) (or, similarly, if Saigon is a city-state and falls under the suzerainty of China [!!!!]), please don't assume that we here at Firaxis think that this is an accurate or desirable representation of history, or a ranking of the civs that deserve inclusion (or do not). One simply cannot rank history in that way - was Sukhothai "greater" than Ayutthaya because its borders - according to a retrospective, nationalist Thai nation-state - were bigger? Certainly not; times were different -
borders as a concept were nonexistent. And to some
phrai in Lanna, it might not matter very much if your suzerain was Mon (800ish-1292), Yuan [meaning the Tai group, not the popular word for Vietnamese] (1292-1456), Laotian (various times), Siamese (1456-1558), Burmese (1558-1775), Siamese (1775-present). You paid your taxes regardless. My point is that if you want history, the Civilopedia has more or less what did happen, whereas your game is your own. And I want to apologise to all the ghosts of those dead kings, queens, emperors, and revolutionaries that we didn't put in the game.
Right this very instant, if you'd like to play as Siam, I think Khmer are your nearest analogue for a Southeast Asian mainland
mahaanajak. (Or one of the great mods out there). Will there be anything more? You're gonna have to wait and see what's in the next three episodes.
Personally, I'm really excited for <ALSO REDACTED>.