Vlingo's speech recognition accuracy depends heavily on the the audio quality of the speech that's sent to Vlingo. Bluetooth headsets vary greatly in the quality of their audio. Some are pretty good; others, not so much. And some are better than others at filtering out background noise. You should expect some accuracy reduction when you use a headset.
In a nutshell, if you think another person would have difficulty making out what you say, so would Vlingo. To get the most out of Vlingo and Bluetooth, you should use a high quality headset and minimize background noise such as music, other conversations, road noise, and wind noise.
Try speaking via your headset in a quiet room, and be sure the microphone is positioned properly per the manufacturer's instructions. This test will give you a baseline so you can compare performance in other environments.
Some headsets adapt to ambient noise levels during the first 5-10 seconds of each use. If you have such a headset, experiment with waiting various lengths of time after the phone vibration before you start speaking. In general, accuracy should improve significantly with a waiting period before you speak.
Vlingo detects that you're using Bluetooth, and it even detects the model of headset that you're using and optimizes it's speech recognition for your particular model.
Just as it does without Bluetooth, Vlingo automatically learns as you use it. You should expect the greatest improvement over roughly the first dozen times you use it with your headset. After that, the learning continues but at a slower rate. If you switch to a new headset, the learning will start again. Don't worry, you won't lose the learning that's already been done for your previous headset.
The downside of waiting to speak is that Vlingo may start to pick up background noise during the pause, and treat it as speech.