Hi longtry!
I'm in Europe but have some ties with Vietnam.
In Europe we have quite a few options and brokers allowing us to invest in worldwide ETFs. Some choose to go through their banks, others go through actively managed funds and then there's the brokers where you can just buy worldwide stocks and ETFs. So for most Europeans on the boards here, it's relatively easy.
Now for Vietnam I can imagine this isn't so clear cut as the part of the population that is interested in/able to invest is much smaller. One of my exes is an American living in VN and I know he invests locally (he had to move his money out of his USA broker once he lost residency), but not sure how and I'm not very keen on contacting him to ask for an internet stranger, sorry :p. Also one of my VN friends' parents have trust funds for their kids that are VN-based, but again I do not know where they have them and am not comfortable asking. I guess all I can say here is that there are actual options!
I am also not aware of any forumites in Vietnam, but I know
@Adventine is in the Phillipines so she may face similar struggles! Most other forum members in Asia that I'm aware of are expats, but they may have useful insights still. I bet there must be some place on the internet where you can find Vietnamese information on investing. I guess you may not be able to find the same kind of super low cost funds, but you may still find plenty of ways to get at least started with international investments.
ETA: you may add to your title that you are in Asia, as it will attract the right members to your thread!