it works ....
I managed to join a domail via VPN connection. I connected the pc to our network (not the applicable domain,connected for internet connection) The pc is running Windows XP service pack 3. Target domain Ran from a Windows small business server 2003 R2.
Firstly i created a vpn connection and connected to test. then joint the domain. After restarting i ticked the checkbox 'log on using dail up connection' filled in the information.then a window came up where i chose the vpn as a connection with the applicable username and password.
It connected and i logged in on the domain.
I dont think this will work with a dail up modem , due to the fact that you will need a online connection already for the VPN to connect on.