VPNs are very common in Vietnam as everyone has seen the Barbie movie here.
I will also add that indie game devs are not going to setup a business in every country just so they can maybe make money from their app there. Maybe a massive publisher can, but are these the game companies we only want providing games in Vietnam? No…and this is why people will just VPN to get.
Are you sure it’s VPNs and not just what it has been for decades - piracy?
VPNs are big because Vietnam is big on censorship. Not nearly to the scale of China, but a lot of popular websites are or have at some point been blocked, so people get used to finding ways around the censorship, which then also lends itself to doing the same for media.
That said, piracy is also very popular there.