VPN TO VPN TUNNEL

We have set up a VPN tunnel between 2 linksys rvs4000 routers. We can get them to connect fine and we can ping the server or map a drive to it, the only problem is we need to join the remote computers to the domain and this is where the problem starts. They fail to join because they cant find the domain controller which is the server I can map to? Please HELP, any info will be GREATLY appreciated. Oh, all the terminals are XP and the server is 2003.