I'm not really sure regarding 2003 QoS, I recall the capabilities there not being very robust. I believe on 2008r2 you can specify by IP - not sure about 2003.
If you can do it by IP then that might be an easier way, as you should be able to set a specific range of IP's for your VPN connections.
So if your network is 192.168.1.x, Set the DHCP for the VPN to 192.168.200-210, then set a rule indicating that anything from 200-210 (you probably need to do each one individually) gets highest priority.
The IP address given by the RRAS VPN is internal existing only behind the RRAS NAT.
Just to clarify, the scenario setup is:
VPN Client --------- Internet --------- Router --------- VPN Server
The router (correct me if I'm wrong) only sees the packets as from the VPN server LAN IP, and the VPN Client's external IP, not the internal IP given by the VPN Server's NAT.
So if I were to assign any range (ie: 192.168.1.200-210) to VPN Clients, it would not matter to the QoS, since the router doesn't see these IPs.
For any VPN packets traveling between the server and client, the router would only see the Client's real IP and the Server's LAN IP. (again, I'm not entirely sure, so correct me if I'm wrong, but it seems reasonable to me)
So it's questionable whether IP based QoS is possible in this scenario.
For IP it is normally under something like 'bandwidth control'. It is for bandwidth throttling but should serve the same purpose as it will prioritize the traffic similar to QoS - when there is congestion, the ones with the higher setting get priority. If there isn't congestion, then it really doesn't matter.
There's is nothing like bandwidth control other than the QoS section on the Dlink.