The OS version you are using is out of date and bound to have numerous security flaws.
You should update to at least v5.6, but as v6.0 final has been released that would be the better way to go. You should remove any custom templates and ideally any non standard add on rpms, so that the upgrade is smooth, and then reinstall add on rpms that are compatible with the newer version OS.
Look in the contribs download section here.
As fas as VPN goes, just enable it in the server manager, and then set up a VPN connection on a client PC and make a connection. It should work OK if you have a bridged modem connection, but with some other configurations you may have to get various protocols configured on external services.
Search on VPN in these forums for lots of information.
Regs
Ray