Think of Vlingo as a person you are speaking to on the phone. If there is a lot of random background noise like street noise, car noise, or background conversation in a loud restaurant, hold the phone closer to your mouth and speak louder.
If the background noise is distinct conversation such as a nearby radio or someone talking near you, Vlingo may not be able to distinguish your words from theirs. Turn down the radio or move a bit further away from the other conversation.
If you are in a very quiet environment, you can speak more softly.