Vlingo always starts listening when you press the activation key. However, it can stop listening either when you press the activation key again or when you stop talking for more than 2 seconds, if it is configured to do so.
To configure Vlingo to stop listening when you stop talking for two seconds, do the following:
1) Go to the Vlingo Home screen.
2) Choose Options.
3) Choose Audio
4) Set the "Stop Listening" options to "On key press only," which means you must press the Vlingo activation key when done speaking to Vlingo. If set this option to "On silence or key press", you can press the key when done speaking or just wait, and Vlingo will automatically stop listening after a period of silence.