You said you have a British passport, flying from Vietnam to Thailand. And you said your bank statement show balance several times of average salaries in Laos, so are you ethnic Loas?
By any chance, are you a dark skin individual?
If so, I believe what she did was racial profiling, although unethical, racial profiling against dark skin customers is quite common and not illegal in many Asian countries. You see all the time that TG ground staff and FAs give preference treatment to "white/light skin" customers. Hotel staff, tour operators, street vendors all do the same, they clearly show preference for light skin color customers. It's the culture there.