When your vpn connection is established your office server and your home computers become two hosts in the same subnet. So, you need to use a vpn (not public) ip address of your server to be connected through remote desktop. And, of course, check firewall exceptions on the remote side.
Also, you can try Remote Desktop Assistant. This is a wrapper for Microsoft Remote Desktop connections. Additionally, it provides connectivity monitoring. You can see whether your remote RDP port is alive or not. Alexander Yarovy - Network Management Software - Downloads