I have been strugling with this issue for the past 2 days. After updating Xubuntu 8 to the 9th core, the VPNC started to act different from what it was before and every time I have connected to the VPNC it was killing the connection for the Internet.
I was discussing this with a friend of mine, much more experienced with Linux, and he told me what was going wrong.
The VPNC removes the default route that connects you to the INTERNET, connecting you only to the private network. If you want to have both of worlds at the same time, you will need to add a new route to the VPN.
Doing this by the GUI is extremely annoying so he did sent me a good article teaching how to do this using the command prompt.
Go to: http://www.gentoo.org/doc/en/vpnc-howto.xml and go more specifically to the #7 item.
Follow what is being told and you are good to go.
Just a note, you need to be a route for that, so use the sudo command to help you out.