Here is a simplified diagram showing my desired setup:
The VLANs seem to be passing traffic over the wds link okay (from RouterB, I can ping all the different interfaces on RouterA). The problem is, when a client connects to either of the VirtualAPs on RouterB, it does not find the DHCP server on the other side of the WDS link and therefore is not able to connect.
If the WDS link was replaced by an ethernet link between the two routers, this would be working. What am I doing wrong?