WMM does not look into frames itself (e.g. at DSCP field). In order for WMM to prioritize packets, you must set priority of packet before it enters WMM enabled wireless link, as discussed here:
Note that VPLS terminating router does not automatically set this packet property - it only initializes "ingress priority" property with value received in EXP bits. So you must add bridge firewall rule that would set packet priority from ingress priority.
So basically you have to do 2 things:
- set priority of packets on your PPPoE AC before they enter VPLS tunnel (this will ensure that your priority value gets sent in EXP bits);
- copy ingress priority to priority on APs (this will ensure that when packet hits wlan interface, its priority will be set to value received in EXP bits) using bridge firewall rule.