This repository hosts some useful scripts for enumerating VPN group names or IDs.
Shell script to enumerate group names from vulnerable Cisco devices by the presence of the Dead Peer Detection payload in the response. See Cisco reference: http://www.cisco.com/en/US/products/csr/cisco-sr-20101124-vpn-grpname.html The script uses ike-scan http://www.nta-monitor.com/tools-resources/security-tools/ike-scan
This Python POC enumerates group names from Cisco devices by differing responses to IKE negotiations. See https://www.trustwave.com/spiderlabs/advisories/TWSL2013-004.txt.
Further details and a guide can be found here: