Hi
VPN entails connecting to a specific location that have a VPN server that you have the correct credentials to connect to it.
It hard to explain details because every issue varies depending on what, and how your system and the remote location where the VPN server is running & configured.
It is impossible to explain in details; while the only detail that you provided is, " i have encounter error 868".
In any case, the issue is best helped by the IT people that are in charge of the VPN server that you are trying to connect to. It is a matter of security and only the local people can handle it.
Maybe this can Help, http://social.technet.microsoft.com/Forums/en-US/w7itpronetworking/thread/1944333e-8b66-41f4-982b-f10954094bd7/
Jack-MVP Windows Networking. WWW.EZLAN.NET