I don't know if we can really connect gambling addiction to a certain nationality, I mean there is no weight on your basis that a lot of them are in the chat box of those sites, maybe they are just friends who really like play at the same time. Also another reason is that no matter what nationality you are you will see that a lot of people in various nationalities are addicted to gambling.
I agree, perhaps they like to communicate to each other and sharing tips in chat box.
I dont see any problem with that and it doesnt mean vietnamese in general are addicted in gambling just because op always spotted them chatting.
I think gambling is part of every culture and not just for a particular nationality.
I even wonder why he would come up with such an assumption. Just because there are some categories of people on the chat, does not make that category an addict, in fact they may even be looking for free money as the case may be.
For example, I do not know about other sites, but for crypto games, there is always a rainbot that gives out free coins based on your activity on the chats, what if they feel they can just chat all day to keep getting the rains from the bots and do not even gamble at all ?