The default installation of virutalmin doesn’t come with additional PHP extensions which some php scripts may need, you have to manually install each php extension compare with easyapache from cPanel which you can install all at once.
Add centOS repo
For CentOS 5
rpm -Uvh http://download.fedoraproject.org/pub/epel/5/i386/epel-release-5-4.noarch.rpm
For CentOS 6