One impression I got from reading Vietnam history is that the Vietnamese regimes historically had a "double-faced" way of interacting with their western and southern neighbors - on the one hand numerous Vietnamese Emperors treated their Khmer, Champa, and Lao neighbors as tributary states of the Vietnam "Empire" (like how Chinese Empires treated Korea and Ryukyu), on the other hand a lot of SEA influences and customs had poured into Vietnam court, not to say that these "tributary states" of Vietnam requires a very different "tributary system" esp. when Siam was involved. As you said, they were/are in this extremely unique position of sitting between the SE Asia and China.
A couple things (also re: "Vietcong"). Do note that I am writing to you as a Southeast Asian historian, not as a representative of Firaxis.
Firstly, be careful with your "double-faced" term there. Nationalist histories have a way of representing their neighbors. In Finnish history (full disclosure: minun isä on suomalainen), the Russians are always aggressive and corrupt and the Swedes are always foolish and pompous, whereas in Swedish history, the Finns are witches and in Russian history the Finns are crazy knife-wielding maniacs. Similarly in Thai history the Burmese are violent and the Khmer are sneaky. AND similarly in Chinese histories the Vietnamese are sneaky. But these are attempts to make stock characters in a story that nations want to tell themselves about how great they are. From the other (Finn, Khmer, Vietnamese) perspective, just because you don't want to be a part of your neighbor's empire doesn't make you sneaky (as the Finns, Khmer, and Vietnamese would tell you). And, of course, from the everyday person's perspective, it doesn't fully matter. So take with a grain of salt this perspective on Vietnamese history from Chinese sources.
About the movement south when the Vietnamese ran over Champa and Khmer people who were there and "incorporated" them into their empire, here, too, it's complicated. Vietnamese is a Khmer-related (distantly, very very distantly) language, and in the northwestern mountains of Vietnam we have the origin points for all the Tai languages. So Kinh (ethnic Vietnamese) are certainly deeply embedded in Southeast Asia, from one perspective, and lived alongside Khmer, Tai, Cham (Tampuan, etc) neighbors for millennia. Taking in your neighbors' culture is unavoidable, even if you're stomping them at the same time.
Secondly, on a larger note re: this discussion, "Vietcong" refers to Communist rebels within South Vietnam who fought against the US-backed South Vietnamese forces. Let's not conflate it with all Vietnamese resisters to the French (or the Viet Minh), or with the Northern Vietnamese Army. They were obviously very close with the NVA, but were not the NVA.
Third, yes, we drew upon some Southeast Asian ideas of statecraft to influence the policies in order to move away from a heavily European card deck. But - as I've said a lot - don't read a lot into it! There's also some Russian references in there!
Finally -- there's some cool stuff coming. I'm really excited about how NFP is developing and I hope that you will be, too.
A Madagascar civ would be nice to see in civ 6 I don't know too much of it though.
*sends an envoy to Antananarivo*
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