Originally Posted by MikeDeltaBrown
3). Maybe this is a semantic distinction but iptables doesn't route traffic. It only drops packets, or not, based on the rules you enter.
This is not correct. iptables does route traffic and routing traffic through a VPN is not something new. A simple google search for "iptables route traffic" will reveal that. iptables can even handle network address translation. You can even get fancy with mangling but I digress.
I'll tackle the OP's problem tomorrow. For now it's my bed time.
@OP for now can you post your iptables config for me to review? Since you're using CentOS your configuration will be located at /etc/sysconfig/iptables. Where are you executing the commands you cite? Is it on the command line or are you running it in a script during start up?