Not sure what model Linksys you had before however Most routers have NAT abilities built in. So they are there own dhcp server and NAT router. Over the past several years, ISPs have teamed up with some router mfrs to have some modem designed with built in routers as well. This also introduces NAT to the modem as well. Think of it as a combo modem. Yes they work ok and are good for the average user, however some folk don't realized this and that understandable from a end user stand point. Not much information is given to end users regarding modems and routers. ISP just expect to install it and customers just want it working. More advanced users are generally more aware of this condition.
You can review this:
Bridge Mode vs Relay vs Acess Point (AP) / Routers vs Dedicated Access Points (AP)
One thing to consider, if modems can't be bridged or if you really want to use the ISP modem has your main DHCP and NAT server, then maybe an external router is not a good solution for you. Rather maybe upgrading to a DAP model wireless AP would be more beneficial. You can turn the 826L in to an AP however you'd be loosing most of the features of the router save the wireless and wired connections. Not sure what you'd like to do here. I think if you can get the 826L into the modems DMZ and then setup the 826L ports for your VPN and test, maybe it will all work out. I've always been leery of the 2wire DMZ though. Seen reports that it's not truly DMZ like it should be.
One thing I don't care for in some of the ISP modems with built in routers. 
Ya, since you have video services thru the 2wire, you'll need to keep that modem or I'd hightly recommend swapping it out for a stand alone DSL modem with out a router built in. Then the 826L would be good to go.
So as a follow-up, I tried the post you linked to, that recommended (since the 2Wire can't be bridged) to use DMZPlus (which forwards all traffic to the 826L, and assigns the 826L the Public IP address). In addition, I set up my port forwarding on the 826L for my VPN ports to the server host. However, clients still get no response error.
I also tried to change the 826L to an Access Point (http://forums.dlink.com/index.php?topic=55781.msg216074#msg216074), but no luck there. I am almost resigned to selling the 826L and just buying a WAP, maybe that is what I should have done in the first place.