I ended up not using DD-WRT to accomplish this.
There seems to be a problem/bug with doing this in the dd-wrt build I have.
Firmware: DD-WRT v24-sp2 (08/07/10) std
Not sure what the problem is but after two days of trying I could not get it to accept connections. There are also a few threads where people have seemingly configured it correctly without getting it to work
http://www.dd-wrt.com/phpBB2/viewtopic.php?p=651839 (cant post more links or I would)
I instead solved this by setting up a dedicated VPN server running on my Hyper-V machine.
I used this guide to set it up under Windows server 2008
Got it to work right away, forwarded TCP port 1723 from DD-wrt to the server.
As a side note:
My DHCP router uses 192.168.1.50-100 and my VPN users get addresses from 192.168.1.200-210. This allows the VPN users access to the network without any NAT translation on the server and by not having any NAT translation they are also blocked from the internet :)