VPN File Sharing

I'm currently using Windows XP Pro (SP2) as a VPN server. I want VPN-authenticated clients to be able to browse local file shares, while ensuring that regular file sharing is inaccessible over the internet.

I tested disabling File and Printer Sharing on the Ethernet device that brings in the VPN connections. While at the same time, File and Printer Sharing is enabled in the Incoming Connections (VPN) settings. When I use this configuration, file sharing fails over VPN when I try to browse to \\server\. When I enable File and Printer Sharing on the Ethernet device, it works (although I know it's probably only working because the client is sending a CIFS request, instead of pushing it through VPN).

Why might this be? I thought VPN encapsulates the packets, then unloads them once they're inside the computer making them the same as a local request. Is the Windows XP VPN service not able to handle this type of configuration?

Thanks in advance!