Thank you for reporting this. It's hard to say why we have many new Vietnamese users. My guess would be that either a particular Vietnamese organisation promoted the StackExchange websites and consequently a group of members decided to enlist, or it is spam-related because each user has a long profile description and, as you mentioned in your question, all profiles link to 1 particular website.
What I find strange is that all users signed up to at least 163 StackExchange sites without performing any other action; no votes, no posts, nothing. Since all profile descriptions follow a somewhat similar pattern they could be created by 1 single user, so at this point I'm inclined to think it's spam. On the other hand some profiles have links to social media accounts which seem to be legit.
Since the users are not violating any rules there is no need to take any action. The links in the profiles are questionable, but profiles hardly get any views especially those of users with 1 reputation point.