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 cooperating network. Other stuff like web browsing should go through the normal data connection.
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).
How to configure this on a recent Android device (>= Android 4.0)?