This blog post deals with VPNC (http://www.unix-ag.uni-kl.de/~massar/vpnc/) : an alternative to Cisco’s linux version client.  If you happy with cisco’s client and were able to install it successfully then you can skip the rest of the blog. 

Most of the information can be found at original sources http://wiki.centos.org/AdditionalResources/Repositories/RPMForge  and http://wiki.centos.org/HowTos/vpnc 

Only issue you might face is installing yum prorities using yum. That is broken in CentOS 5.3 (atleast til I tried). So here are the steps involved. 

1) Install Yum Prorities

Install prorities using rpm instead of yum.

wget http://mirror.centos.org/centos-5/5.3/os/i386/CentOS/yum-priorities-1.1.16-13.el5.centos.noarch.rpm    
rpm -ivh yum-priorities-1.1.16-13.el5.centos.noarch.rpm
                     
2) Check if plugin is enabled (see http://wiki.centos.org/AdditionalResources/Repositories/RPMForge for details)

nano /etc/yum/pluginconf.d/priorities.conf

3) Update Proirities for repo files

nano /etc/yum.repos.d/CentOS-Base.repo

4) Install rpm forge

wget http://apt.sw.be/redhat/el5/en/x86_64/RPMS.dag/rpmforge-release-0.3.6-1.el5.rf.x86_64.rpm
rpm –import http://dag.wieers.com/rpm/packages/RPM-GPG-KEY.dag.txt
rpm -K rpmforge-release-0.3.6-1.el5.rf.x86_64.rpm
rpm -i rpmforge-release-0.3.6-1.el5.rf.x86_64.rpm

5) Update prority for rpmforge repo file

nano /etc/yum.repos.d/rpmforge.repo

6) Update yum to start using priorirites and rpmforge as source.

yum check-update

7) install VPNC

yum -y install vpnc

Now vpnc is installed.  Follow http://wiki.centos.org/HowTos/vpnc to know how to use it.