Q: I can't use Vietnamese keyboard (VNI especially)

VNI seems to work OK for me.  Can you provide details on exactly what is not working for you?   In VNI, what do you get when you type truo72ng in TextEdit or here with Safari?  I get trường

Try creating a new user account and see if you have the same problem when you are logged into that.

I had the exact same problem. What I did was to go to System Preferences/Language & Region/Keyboard Preferences/Input Sources to delete the "old" VNI, then restart my Mac, then re-add VNI and it worked. Give it a go and see if it works for you. Cheers, Leo