The default ibus installation in arch doesn't include gtk3 support. You will have to compile the source. This can be done by pull in the ibus-git package from AUR with yaourt.
When you are ask to edit PKGBUILD, go to the configure section and add –enable-gtk3 options, install it and all is set.
The problem is ibus-unikey, which is – up to date – the most popular Vietnamese input method in linux will have to be recompile to work with the new ibus. Don't download the source package it's oudated. Do a git pull as instructed in http://code.google.com/p/ibus-unikey/wiki/CompileDevSource:
git clone git://github.com/mrlequoctuan/ibus-unikey.git ibus-unikey cd ibus-unikey ./autogen.sh --prefix=/usr make sudo make install