The only thing I know about VLANs is Rules #1 and #2 of the Mikrotik Club, which may or may not be useful in your case, still I would respect them, to avoid possible issues:
http://forum.mikrotik.com/t/network-traffic-gets-slower-when-adding-vlans/178615/1
One explanation:
http://forum.mikrotik.com/t/vlan-configuration/176351/1