If I understand you correctly, you're now just trying to figure out how to assign a DNS server to a specific VPN client? Assuming you're still using DD-WRT's VPN server, I believe the actual PPTP daemon it uses is called PoPToP. You could check its manual to see if there is an option you can set to do what you want.
https://www.systutorials.com/docs/li.../5-pptpd.conf/
https://www.systutorials.com/docs/linux/man/8-pptpd/
I just gave it a once-over and didn't see anything. Looks like you're running up against the limitations of the technology you've chosen. I agree it's time for a change.
I'd strongly suggest the EdgeRouter X, which supports a mix of PPTP if you need it, as well as IPSec and OpenVPN in a $49 package. Happy to help configure that if you decide to go down that road.