Am I correct in assuming that you have
2 routers(SH for normal traffic and most devices, dd-wrt for VPN and your devices)?
If this is the case you not have change any config on the SH just connect the dd-wrt to the SH and run the VPN on dd-wrt.
Now on the devices you want to go through the VPN change these devices default gateway to the IP of the open-wrt IP. This will send all traffic destined to outside your network to use the VPN router.
Regardless if the device is connected either by physicaly/wifi to the VPN router it will still be able to send the traffic through the VPN once the VPN router is accessable from the SH network.