I have been looking into this myself but from what I am reading, DD-WRT doesn't support IPSec, which seem completely barmy as it is and has been the de-facto standard for VPN tunnels (especially site-to-site) for ever and day. They seem to prefer OpenVPN which I suppose has its usefulness, but is simply not supported in other routers.
However, you might want to switch allegiances and look at OpenWRT. They DO support IPSec (and even Netgear DG834 routers, which DD-WRT doesn't).
FWIW, this post (http://www.dd-wrt.com/phpBB2/viewtopic.php?t=141285) lists a poll for users to vote for the implementation of IPSec in dd-wrt. Unbelievably, this was started over 3 years ago and the majority of people who have responded wish it to be implemented.