Vlingo returns the warning "I didn't catch that. Please try again." when it isn't able to discern what it heard. This may happen for a variety of reasons, particularly with lots of background noise or not holding the side key for the whole time you're speaking.
To recover, click OK and speak again. Begin speaking immediately after the Listening... prompt is displayed, and release the key immediately after you finish speaking. For best results, speak clearly but naturally.