To type in Vietnamese in general and Ubuntu Linux in particular, You are more popular choice as Unikey, Polypody… but really, I find it somewhat Unikey errors and inconveniences when typing it on underlined below and click on Where is he that he copied it from being knocked out of the way, there is no use scim ever. Finally, I see Bogo (Percussion) is very stable.
Settings: You see here for more details, very detailed and complete.

Vietnamese input on the browser (firefox and chrome, Chromium) by Add-on AVIM. AVIM is agile percussion allow text input Vietnamese naturally you will not even that I'm using it. It comes with all the features you expect from a modern IME, such:

Support for the four major input methods: Telex, VNI, VIQR, and VIQR *
Automatic spell checking and correction rule diacritic
Sensible keyboard shortcuts (such as Ctrl Alt V) for configuring AVIM as you type
Full support for Web applications like Gmail and Office Web Apps

Settings:
– AVIM for firefox
– AVIM for chromium, chromium