The Offer Remote Assistance feature does not work when you use a VPN to connect a Windows Vista-based computer to a corporate network
Consider the following scenario. You use a virtual private network (VPN) to connect a Windows Vista-based computer to your corporate network. After you connect to the corporate network, you experience an issue with your computer, and you contact your corporate helpdesk to resolve the issue. You provide the VPN IP address to the helpdesk agent. In this scenario, the helpdesk agent cannot use the Offer Remote Assistance feature to connect to your computer.
For more information on this issue, including potential causes, workarounds, and resolutions, see: Microsoft KB Article KB954386.