dd-wrt is so fucking dumb! because of your problem and one i previously worked on (basically routing internet data between two routers) i knew how stupid it is. anyway after 1 hour of working i figured something out for you.
First setup the VPN client as you did in services tab and hit save and apply don't input the command script they provided!...
go to setup-->advanced routing and open the routing table refresh for a while you should see the IP of the VPN server routed to 192.168.1.254 if you did then VPN is UP if you didn't then you have something wrong with the VPN setup... anyway if you saw it then proceed to step 2
Step2: you need to add the following scripts to the stupid dd-wrt
Save this one as starup script
#!/bin/sh
sleep 50
VPN=$(/usr/sbin/nvram get pptpd_client_srvip)
/sbin/route add -host $VPN gw 192.168.1.254 dev vlan2
/sbin/route del default
/sbin/route add default dev ppp0
iptables -t nat -A POSTROUTING -o ppp0 -j MASQUERADE
save this one as firewall script
iptables -I FORWARD -i br0 -o tun0 -j ACCEPT
iptables -I FORWARD -i tun0 -o br0 -j ACCEPT
hit administration then reboot when router is on enter the advanced routing then open routing table and refresh for a while first you should see the VPN server ip routed to 192.168.1.254 keep refreshing it should after a while it should delete the default route 0.0.0.0 192.168.1.254 when that happens your data is routed through the vpn hopefully
@stygmata what does isp nat have to do with all this lol, no it works fine