Managing personal Outlook.com account settings, security, and privacy
Hello, thank you for reaching out to Microsoft Q&A.
Do you get any specific error message when trying to switch your region to Vietnam?
Open an incognito tab on any web browser and sign in with your Microsoft Account here -
https://account.microsoft.com/profile
Change Country/Region, then Language and Regional Formats.
Then, Click Save.
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Then, go to https://outlook.live.com/mail/0/options/general/timeAndLanguage
Change your Language (Country/Region) and Time Zone.
If you run into any issues, let me know.