VN is an abbreviation for Vietnam. Further details can be found here: https://www.abbreviations.com/VN.
While the 2 letters language code for Vietnamese is VI and 3 letters language code for Vietnamese is VIE. Source: https://www.loc.gov/standards/iso639-2/php/code_list.php. Because people are more familiar with the VN term, I suggest you use @onboarding-vn.

I love your project and it is similar to what I have been doing. Steemit culture is somewhat different than Facebook, Reddit ... I've found that some new users, especially from non-English speaking countries are having trouble integrating with Steemit. They are lost and in some worse cases get flagged. Most of them just abandon their accounts and leave Steemit. I strongly believe that by giving them a proper support and guidance will reduce the number of new users getting flagged too.

I wonder if you have a Discord channel?