how can I use different apple ID in the same device ?

You would need to sign out of the Hong Kong Apple ID in Settings->Account(top of settings)->Media & Purchases->Sign Out, and sign in with a Vietnam account.

Do not that doing this will then lock the Hong Kong account from being used for 90 days on the device.

If you are permanently residing in Vietnam now, and want to change your current Apple ID store to the Vietnam Store see here: link-> Change your Apple ID country or region - Apple Support

Note all the changes that entails with your purchased content.

As I said, no. Changing Apple IDs and downloading apps or media with one Apple ID will lock the other Apple ID for 90 days. So it cannot be logged into the device again for 3 months. In other words, no, you cannot change Apple IDs frequently.