I have VPN configured using Mac's native vpn support. It is working when I am using WiFi or ethernet connection but VPN becomes unavailable (drop-down menu says "VPN is not configured") when I'm using a 3G dial-up dongle.
When I open Network Prefrences, the location is set to Mobile Broadband and the VPN entries are not listed at all.
When I select Location: automatic the Network Preferences window becomes very sluggish,but shows all VPN entries. I then hit connect on the VPN and nothing happens.