I've installed the ibus-unikey package, but I can't use Vietnamese (Unikey) (Ibus) Method. It's not available. I tried to remove and reinstall the package, but this problem remains.
Is there any way to fix it?!
My friend installed Ubuntu 15.04 64 bit. Then he installed Vietnamese (Ibus) (Unikey). There's no problem with his computer.
I think the cause of my problem was updating my Ubuntu from an older version to 15.04. The system reported some error, so I tried to restart my computer and continued the update.
After reinstalling 15.04, this problem disappeared.
You need to log out and log in again.
Edit:
I now understand from your comment that you did. Then the next advice would be to open System Settings -> Language Support and make sure that the selected "Keyboard input method system" is IBus (and relogin if you need to change it).