Hi,
Thank you for writing to Microsoft Community Forums.
I understand that you see a text box when using Vietnamese keyboard.
Based on my research, certain keyboard will show a text box when typing, and currently we don’t have the option to remove the text box as this is by design.
However, I understand your requirements as this can be important for you. I would encourage you to provide a feedback using the Feedback Hub app on Windows 10, so that our engineering team can look into this for you.
Regards,
Prakhar Khare
Microsoft Community – Moderator