Remote access Team Foundation Server through VPN

If you want to allow your Team Foundation Server clients remotely access the Team Foundation Server, you can implement Virtual Private Network (VPN). 

The initial release of Visual Studio Team Foundation Server supports only Integrated Windows authentication in Microsoft Windows operating systems. But, Integrated Windows authentication is not the best choice in Internet scenario. There are some limitations on the use of Integrated Windows authentication because of the proxy servers, firewalls, and trusted connections may create limitations on.

Because of this, you will have problem to access Team Foundation Server through a proxy.
In order to alow remote access to the Team Foundation Server, you have to maintain a connection between the client and the server by using a VPN if you have to access Team Foundation Server from outside the local intranet.

Other option if you do not want to use VPN is that you may want to expose the Team Foundaiton Server to the Internet and let the client to access it through an encrypted connections. For example, you may use the the Transport Layer Security(TLS)/Secure Sockets Layer(SSL) security protocol and let your clients to accesss the server through Secure Hypertext Transfer Protocol (HTTPs).