Huawei 3G/HSDP USB Modem and VPN interworking issue

Hi, New Mac convert here and I'm loving the Macbook Pro. However I have a great USB modem that allows me about 2Gb speeds in the right mobile coverage. This works with the Mac for most things, but when I try and run my Cisco VPN client over it I get an error message saying the VPN sub-system cannot detect an active internet connection (or something like that). The internet access works fine for browsing, and the VPN client works fine with a cable or WLAN so it appears to be something between the two of them.

Any thoughts gratefully received