You may get lucky searching the forums for additional help on configuring DHCP for mutliple VLANs with Pi-hole's embedded dnsmasq.
slowlearner's approach in Extra DHCP Server Options - #5 by slowlearner seems quite detailed and helpful, if a bit verbose at the beginning. If in a hurry, just skip the verbose account of his ordeals to arrive at a solution and start reading at "WHAT YOU’LL NEED TO DO:".
I haven't tested or reenacted any of this, but it seems carefully compiled and overall conclusive to me. 