One of the big gripes that people have with Ubiquiti is the lack of support for Virtual SSIDs. Here is a step by step tutorial for setting up VSSIDs on Airos 5.6 devices with Vlans back to the upstream router. Please note that you will not be able to use Airmax when you have Virtual SSIDs.
This tutorial is based on information from the Ubiquiti Forums, specifically this post by AnubisSL.