I know Upmost has already replied to you but still I wanna jump in and add my cents.
It’s evident from the East Asian architecture, the HP bonus on archers / cavalry archers, the in-game language, and the AI player names that the Vietnamese civ only covers the Dai Viet kingdom and not Champa.
From both a historical and a gameplay POV, I think the Chams deserve to be a new SE Asian civ. From a historical POV, for much of the AoE 2’s time frame they were independent from the Vietnamese and not only that also fought many wars against the Dai Viet. They spoke an Austronesian language related to Malays and Visayans but different from Vietnamese which is an Austroasiatic language. They were also skilled sailors and pirates and often preyed upon passing merchant ships in the South China Sea. During their heyday in the late 12th century, they practically dominated much of the trade that went over the surrounding seas, giving rise to the name Sea of Champa for what we now call the South China Sea, which is a relatively recent term.
From a gameplay perspective, it doesn’t matter whether you focus on their prowess in navy or in piracy, or maybe their usage of Austroasiatic crossbows and the employment of women soldiers like Upmost already mentioned, they would play out very differently from the current Vietnamese civ.
I can share with you my take on the Chams, my focus being their prowess in navy and raiding, with the UU being the Mangpung which is an amphibious raiding unit. Overall they would have strong navy and above average archers and infantry but weak siege and weak stable.
User blog:MNOPSC1b/Chams (potential new SE Asian civ for AOE 2) | Age of Empires Series Wiki | Fandom
Just like you said, the only issue preventing their introduction into the game may be political. However, from what I heard (I’m not Vietnamese so please don’t quote me on this), the Vietnamese government seems to be more lax or more tolerant on these issues compared to the Chinese government, so there’s still a chance for the Chams to be introduced.