vpnc-brute is a modified version of the VPNC client (https://www.unix-ag.uni-kl.de/~massar/vpnc/) that add capabilities to automatically check a combination of user and password . The source code has been patched to work without user interation and a bash script has been created to run the vpnc client.
# bash vpnc-brute.sh
./vpnc-brute -h <gateway> -g <group-id> -s <secret> -f <file>
-h: IP of the remote gateway
-g: group id
-s: group password
-f: file of username:password
The code can be downloaded at: https://github.com/inode-/vpnc-brute