Actually this needs to be done on the switchport configuration on your switch and not on the IAP itself.
Let's say I want to trunk the following VLANs (100, 200, 300, 400) and want to pull a DHCP IP off VLAN 200 for the IAP.
I will need to set VLAN 200 as the native VLAN on the switch so the IAP can pull it's IP address off that subnet. Keep in mind in an IAP environment you will have to replicate this port config to every switchport housing IAP's. Then in the IAP cluster you can configure the VC as a static IP within VLAN 200.
Basic configuration for Cisco Catalyst:
switchport trunk encapsulation dot1q
switchport mode trunk
switchport trunk native vlan 200
switchport trunk allowed vlan 100,200,300,400
After your IAP's have booted up and received IP's from the correct VLAN (200) ; you can start assigning which VLAN's you want to drop clients on when they associate to SSID's availale on the cluster. You will do that by going under the SSID, under the VLAN tab, selecting Static, and then entering the VLAN tag/number you want to place the client on. Obviously the VLAN needs to be accessible to the IAP (allowed across the trunk).
Hope this helps!