Setting up VPN through Cisco 1721 and firewall

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I'm fairly new to routing, so any help would be greatly appreciated.

Network is configured as follows:

Inside Network -> Watchguard Firewall -> Cisco 1721 -> Internet

I need to setup VPN access using a Windows server, on our inside network (10.10.1.0). All NAT is done through the Firewall, and then routed to the 1721. We have a range of public IP addresses we can use.

I've set up VPN on the server, and setup a port forwarding on the firewall for 1723 to our server (10.10.1.254).

What I need help on is this: What do I need to do now so that somebody attempting to connect from outside has the VPN connection route to the server?

If I understand correctly, I need to have the 1721 route any traffic using port 1723 to our Firewall public IP, which would then forward that traffic to server using the internal address. I had thought about forwarding only a specific IP address to the firewall address, but I can't tell which IP addresses the firewall is using when it NATs, so I think it needs to be on a port or protocol basis.

If this is correct, how would this be done on the 1721? I understand the basics of navigating IOS, but I don't know the correct way to implement it (or even if what I'm thinking is correct).

Thanks, J