Actually it does work while on vibrate, but it's like the blind leading the blind. If you are able to time out when the "tones" play, to when you are supposed to speak your commands, it will work.
For some reason, when you switch it to vibrate only, you won't hear the call waiting tone, or the voice command tones. It took me 3 hours to figure out that I had my ringer off. So then i began to play with it to determine what the deal was.
Definitely need to keep the ringer on low in order for it to work.