Does Facebook have rules that are consistently applied to every country? AFAIK, Facebook complies with the local governments' requirements everywhere it operates even though they will occasionally publicize some resistance.
Facebook isn't complying with any local law here, and the behaviour in question isn't isolated to users in Vietnam. They are turning a blind eye to avoid retaliation for applying the same rules they'd apply elsewhere.
https://www.washingtonpost.com/news/global-opinions/wp/2018/...
The law doesn't explicitly state allowing troll bots, but it does have extremely strong opinions about how Facebook operates in Vietnam.
That sounds like a rule that's applied consistently, to every country. And yes, I know it's weasel words that mean exactly the opposite of what they appear to mean.