use VPN to connect to only one software in the remote desktop

I am extending my desktop application to be available for salespeople on the field using a VPN connection. when the user try to connect to the desktop machine it should allow only the given software to be accessed by the salesman. in windows remote desktop we can configure an application to load when connecting to remote desktop. but i could not see it can restrict to only one. Could anybody please help me to figure out how it can be done.? i am not all familiar with ssl, tunnel, routing etc.. 🙁 so if it could be plain it would really be helpful

Thanks a lot