I thought about what
@DogeEnricoDandolo said about guerilla tactics of Vietnameses being part of their cultural heritage and that a unique encampment is kind of counter-intuitive to this ability, but let's think outside the box (something FXS tended to do more with NFP).
WHAT IF ! You
have to build all your specialty districts on passable features EXCEPT for the encampment that must be built on open terrain, and the encampment district + every building in it allows your units garrisoned on a district on a feature to gain additional bonuses to defense? To represent the fact that the encampment is in itself kind of weak (a trap they lure Mongolians in) while the rest of the army is staying steady in their marshes/jungles/forest in optimal defense position, making their encampments weaks but that make stronger other districts?
Could lead to interesting military tactis and make Vietnameses a truly difficult country to invade (which has been the case until... well, the French, but tbh, French aren't real people, they eat frogs and snails, so a humid and hot climate is not that bad).