It seems that when you add an incoming connection in Windows 8 for VPN connections, the connection is automatically placed in the “Public” category which subjects it to the most stringent firewall rules and won’t let you connect to shares on the machine.

This can be altered using the Group Policy editor.

Start GPEdit.msc and you’ll find it here:

Computer Configuration -> Windows Settings -> Security Settings -> Network List Manager Policies

Right-click the “RAS (Dial In) Interface” and change it from “Not Defined” to “Private”