Well known trusted vendors like Verisign are "needed" if you use webvpn or ssl connections to your CVPN only if you need to have your VPNC certified as a trusted device, in your case since your concentrator is not using webvpn nor ssl and you are only using VPN Client, you don't really need a certificate at all. Unless your vpn clients start authenticating using PKI (in which case you would not need a Verisign cert either)
