VPN is not needed to do this, although what you recommended will work. If
you want to connect to a server in your office, open a TCP port 3389 on your
firewall to a server you want to manage, then connect to it via your WAN IP
Address using Remote Desktop Client. If you need to manage others, you can
open up more sessions to those boxes from the original session. You could
also look at a product like WTSGateway from Terminal-services.net, which acts
as an RDP router so you can connect to a bunch of servers via one IP Address.