If the vibrate mode on your iPhone stops working and you are sure that you have vibrate turned on, one thing to try is giving your iPhone a couple of slaps with your hand. You don't want to bashing the thing as it is an expensive piece of electronics, but apparently the vibrate mechanism can become stuck. And if yours isn't working because it's stuck, jarring it a bit can unstick the mechanism and get the vibrate working on your iPhone again.