Say you only want to route the traffic of a single application through a VPN.
(Because the application lacks proper encryption support or the remote service is only available on a cooperate network.)
Other stuff like web-browsing should go through the normal data connection.
How to configure this on a recent Android device (>= Android 4.0)?
Concrete motivation: Unfortunately, quite a few VoIP gateway providers do not provide encryption of data (voice and signaling) - thus using a VPN is necessary (at least when connected via WLAN). One provider (sipgate.de) even provides its own VPN (and no other SIP/RTP encryption methods).