Any QoS on MPLS/VPLS, based on MPLS QoS tags, has to be handled in L2 (for example NV2 QoS).
L3 QoS (queues), do NOT work on MPLS traffic in RouterOS.
This is due to how packet flow in handled in RouterOS, see this: http://forum.mikrotik.com/t/new-packet-flow-diagram/66252/1