What gave you the idea they are foreign? On the contrary, it's relatively hard to open a bank in Vietnam as a foreigner.

This is because Vietnam is in the process of a massive modernization of their banking and tax system. They introduced biometric ids for citizens a few years ago, then around a year ago they required everyone with a bank account to link it with their id.

These are the bank accounts that didn't get linked. I'm sure a big chunk of them are just people's old barely used accounts that they couldn't be bothered going into a bank and registering.