Devices using iOS 6.1 and later with VPN On Demand configured to "Always" will behave as if they were configured with the "Establish if needed" option. The device will establish a VPN On Demand connection only if it is unable to resolve the DNS name of the host it is trying to reach. This change will be distributed in an update later this month.
For now Apple only suggests turning on VPN manually in Settings > General > VPN; however, the company says it will 'address this functionality with alternatives in a future software update.'
Read More [via Loop]