If you have a computer that you want to join a domain and the computer is connected via VPN you need to do the following steps. Hopefully this will helps someone, unlike me, I had to figure this out myself after search google high and low with no help.

1. Connect your site to site VPN.
2. Configure your VPN appliance to use the targeted domains IP as the DNS
3. Join the domain form the computer properties the usual way except, instead of using the pre-2000 domain name ie "Network", you need to use the fully qualified name ie "MyNetwork.com"

Now if newly joined PC is on a different subnet than you Domain Controller you will need to do this in order to access internal servers/PCs via a remote connection by name:

Go to <root system Dir>\System32\Drivers\etc\    Look for the file name "hosts". In that file you will need to map your Servers/PCs name and IP address.

Keep in mind if you join your PC to a domain while at the domain site and not thru VPN, then take it offsite, you will not need to do these steps. This is only for joining a PC connected via VPN that has never been joined before.

Ed K.