The script /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
/etc/vpnc/vpnc-script-disconnect-action
and
/etc/vpnc/vpnc-script-post-disconnect-action
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.