This is a routing question, so using iptables may not suitable. Here we will use iproute2, which, fortunately, included with DD-WRT.
Suppose 188.8.131.52 is the IP address of the default gateway of the DD-WRT (you have to figure it out, may be by ways of disable OpenVPN so that the default gateway will be set to normal and issue a route -n command to see the normal - before OpenVPN-get-connected default gateway).
Now set up another route table like that (we will use table 10):
# ip route add default via 184.108.40.206 table 10
And set up rules so that traffic from some IPs in the LAN will get routed using this new table:
# ip rule add from 192.168.0.0/24 table 10
Or from individual IPs:
# ip rule add from 192.168.0.3/32 table 10
# ip rule add from 192.168.0.5/32 table 10
You can verify with:
# ip route show table 10
# ip rule list
Read more here on startup scripts on DD-WRT if you want all of these to survive on reboot.