Wait, so you're saying that the distinctive cultural achievment of Vietnam is... to borrow cultures from all around their neighbors?
It could be extremely interesting as a civ ability, don't you think? Like:
In addition to Vietnam's CUA, UI and UU, each time a foreign culture became dominant in Vietnam, they gain the ability to build the UU, UI and use the CUA of the foreign civ
At the beginning, it would be kind of weak, but as the world advance and more culture expand their tourism, Vietnam will gain more and more bonuses... Yes, it's dangerous because one of your opponent might win a cultural victory this way, but it would be like Kongo: you might let another civ win the Religious Victory but you need it to shine really bright.
Just a silly idea thrown here. Let's see what Vietnam will become (even if, not being an American, I never saw Vietnam as a warfare civ, but more a merchant or even a cultural one, but I have to admit my knowledge is heavily based upon the Tran-Nhut novels with the Mandarin Tân, while being fictions, are still very accurate historically speraking, with lots of sources at the end of the novels)
I don't think that's how culture works in this game.
Moreover Vietnam was heavily influenced by the "four occupations" system originated in China, which divided the common citizens into 4 main classes: intelligentsia, farmer, craftsmen and merchant. Intelligentsia is on the top of this hierarchy, farmer class is lower than the intelligentsia, craftsmen class is lower than farmer class, and merchant is the lowest class in this hierarchy.
I think one of the previous poster's gripes about historical slights in Vietnam-Thai relations should be a decent hint, but Vietnamese were not a very peaceful nation. Their history is basically one of continual expansion. Heck, they had a 200 year long civil war and even then the separate kingdoms (nominally under the same emperor) kept expanding their territory. Likewise, I'm not so sure about the whole culture borrowing portion. We had mentioned previously about the elephant tactics and symbolism they acquired from the Champas, but they also carried out a mostly successful and through ethnicide of those folks in the process. They do have a little more of South Asia in them than the Chinese but they were just as adept at sincisation of any barbarians they got under their control.
Vietnam's culture, philosophies, thoughts... were heavily influenced by China, then later it was also influenced by France when Vietnam became its colony, but after Vietnam became independent the influences from France faded, making place for influences from the Soviet Union as well as the US...
Lý and Trần dynasties of Vietnam were influenced by Champa to some extent, evidently through the usage and reverent of elephants, the decorations on armors, and Dai Viet (Vietnam's old name) regularly brought back artisans and Apsara dancers whenever it successfully invaded and looted Champa - the royals loved to watch the Apsara dance... Nowadays influences from Champa on Vietnam had vanished, but back then it was different.
Above were some details I remember having read in forum threads involving modding of games of Total War series, and it always fell to Vietnamese team to make the Champa faction, in addition to the Vietnam faction.
It should be noted that Vietnam only began sinicizing the populace of territories conquered by Vietnam by the time of its final dynasty: Nguyễn dynasty. Before that, it was simply massive land grabs, and the Cham people was even allowed to maintain an autonomous region inside Dai Viet. But by the time Nguyễn dynasty was established (through winning the civil war), that was the time the influences from China and Confucianism peaked.
At that time China was conquered by the Manchu, who established the Qing dynasty. The Nguyễn kings had the funny idea that since China was already conquered by Manchu, so Chinese people should only be referred to as Qing citizens, meanwhile Vietnamese could be referred to as Han people, because apparently Han people back then was a concept which denoted people who followed and practiced the culture of China, instead of limited to ethnically Chinese like nowadays. The kings claimed that Vietnam was the true inheritor of the pre-Manchu China culture, instead of China itself which was ruled by foreigners.
Beside referring to Vietnamses as Han people, Nguyễn kings also referred to Vietnam as the Middle Kingdom - the name only reserved for China previously.
So yeah, only since that time would Vietnamese attempt to sinicize the Khmers in Southern Vietnam (which originally belonged to Cambodia) as well as in Cambodia when Vietnam invaded and annexed those regions, and also completely annexed the autonomous territory of Cham people as well.