Routing all traffic except a few IP-ranges though default gateway in Mountain Lion
If you configure your VPN server to allow split tunneling, this will work without any need to configuree your vpn client or the network on the Mac. 


http://www.cisco.com/c/en/us/support/docs/security/anyconnect-secure-mobility-client/119006-configure-anyconnect-00.html



  Split-tunneling is used in scenarios where only specific traffic must be tunneled, opposed to scenarios where all of the client machine-generated traffic flows across the VPN when connected. Use of the AnyConnect Configuration Wizard will by default result in a tunnel-all configuration on the ASA. Split tunnelling must be configured separately, which is explained in further detail in the Split Tunnel section of this document.