If the host Windows 7 machine has a VPN connection, can that be seen/used by an application running in XP Mode?
I actually have the reverse problem whereby I need to run my secure VPN software in XP mode and can't access that tunnel from Win7. But to answer your question, it depends on the VPN software. If your software creates an exclusive tunnel, then XP should be using the same tunnel. However if your tunnel isn't exclusive (that is if not all data goes through the tunnel when it's active), then you may have some problems. It may be as easy as telling your VPN software to extend the tunnel to XP.