It what must be a relatively common problem (but seemingly not documented), I came across the issue of attempting to VPN from one NATed network to another. As both networks were utilised the 192.168.0.1 - 192.168.0.255 range, attempts to VPN caused a lack of external network access and prevented access to internal resources on the remote network.
The solution was to change the server network (where the VPN was) to utilise the 10.0.0.0/16 range so that gateway addresses and DNS addresses would not overlap.
In setting up the VPN connection on OS X, it also required the DNS server on the VPN connection to be manually set to allow address resolution for external site access.