I haven't used VirtualBox, but with e.g. VMWare it's possible to bind the virtual network interface to a specific network card such as the Wi-Fi card. Perhaps this is possible in the VirtualBox settings too?
The other applications running on Windows 7 should use whichever interface Windows thinks is best, which is probably the first interface that was active during boot (probably LAN) - at least that's how it works under Windows XP. It should be determined by the routing tables which can be queried and changed with the ROUTE command in a command prompt.