Is it possible to have a 2nd router specifically for VPN purposes?

Yes, most likely. There are good reasons to do this, like the client does not have software VPN capability. The second "router" is just a VPN node/endpoint and forwards traffic using say PPTP or OpenVPN. Now like most things DDWRT like, it might brick on you or not work.

The other option is to used a wired Windows box and share the VPN using connection sharing, broadcasting it as WIFI.

A VPN is a secure connection between to specific end-points. If you are a home worker accessing your company's corporate network, one end point can be your computer, and the other end could be a gateway onto the corporate systems.

Another use of a VPN is to protect portable devices when they are accessing open WiFi (with no encryption) at places such as an airport in Starbucks to provide the protection that the WiFi does not provide itself. In this case, the VPN is connecting to an end point in a proxy server, that then accesses your target website over the Internet without any further protection from the VPN.

Many people refer to VPNs when they really mean they are using a proxy server. Often proxy servers are used to make the user appear to be in a different location from their true location. This can be contrary to the Terms of Use of many sites, and if it is used to access movies or TV shows that otherwise are not available, can be classed as copyright theft with the possibility of prosecution, a criminal record, a fine or even imprisonment.

Now to get back to your question. Why do you want to use a router? Why not install a VPN and its proxy settings on the computer. There should be no reason to have a separate router with VPN firmware. A dual boot computer is probably better than having to change network connections between using a VPN and not using a VPN.

I hope this helps.

You can just set the VPN connection on your one computer... you do not have to use a router for VPN purposes. That applies even if you want a VPN "server" or use your computer as a VPN client.

If setup as a VPN server you just forward the ports need to the VPN computer.

A second router will just add unneeded DUAL NAT issues to your system and make it very difficult to setup properly. Yes it "CAN" be done... however, based on what you say... you don't need to even try it! A full understanding of networking would be needed and the setup is NOT SIMPLE.