There are many solutions that help typing Vietnamese on Linux (RedHat/Fedora/CentOS). One of those is SCIM. It is very simple to install Scim package.
Follow these steps to install scim:
# yum install scim scim-libs scim-tables scim-bridge scim-tables-vietnamese
(you can type yum install scim* to look for other languages)
SCIM defaultly supports Viqr, you need to download and install the following packages for Telex and VNI
Download the newest file from: Scim-Vietnamese
# rpm -ivh scim*.rpm
Select System → Preferences → More Preferences → Input Method
Logout then login again. Now you can easily type Vietnamese (use Ctrl + Space to switch)
Have fun!
Have fun!