/etc/vpnc/vpnc-script is called on various events, including connect and disconnect.
This sounds like you are using ubuntu or debian. If you are, the scripts
are called for the relevant events. You can create those files if they don't exist, and put whatever logic you want in there.
If you aren't using a debian-based distribution with these sub-scripts, you can modify
/etc/vpnc/vpnc-script directly to add whatever logic you want.
If you are going to have logic to call vpnc-connect again, I recommend disconnecting that from the disconnect script somehow so you don't end up in an infinite loop.