I was having the same problem. My Sonicwall VPN connection used to work fine on both physical and virtual machines within my LAN. This was until Apple's firmware upgrade 7.3.1 came out and screwed everything up. A simple test (connecting my computer directly to the cable modem bypassing Apple's router) proved that the Airport was in fact the problem. The solution? I don't like it but it's simple. Just downgrade your Airport base station to firmware 7.2.1. This is very easy to do from Airport Utility - Click "Manual Setup" - Sumamry tab. Hover your mouse over the "Version" label and click on it. You will get a drop-down with firmware options to upgrade/downgrade to. Use 7.2.1 because is the last one that works. I tested 7.3.1 and 7.3.2 and none of those work. Maybe a future one will. Hope this helps.