The virtual controller IP you speak of is used to have a static IP to manage a multi-Instant AP network. It is assigned to the IAP that becomes a Virtual Controller in the event the initial one fails. There is no way to set the mask or gateway for this setting.
To give the IAP itself a static IP, click the IAP you want to change under the Access Points tab. Click Edit. You can change the name and IP (mask,gw,dns,etc.).