Sharing Files over a VPN may not be straight forward depending on your network setup.
Over a VPN, local LAN settings won't be visible on both connecting networks. Apple Bonjour and Windows Filesharing works on a local level.
If you have Active Directory, this won't be an issue. The AD server acts as an intermediary.
Having a dedicated file server, or AD server will help resolve this issue. Alternatively you can manually type IPs, the VPN can find the remote PCs that way.