I had this problem with my phone as well. It didn't matter if I went into my settings and changed it to vibrate or not, it would not vibrate at all. I finally decided to download an app from the droid market called MineVibrationSetting, and only then did it start to work. However... when I wanted to have a ringtone again, I had trouble getting my phone to not vibrate instead of ring. LOL. Go figure!