If you find Keen's vibration is distracting to others in quiet situations, such a classroom or library, Keen has two settings you may adjust.  Open your app, go to the Device screen, and tap Manage next to your Keen.  On the next screen, tap Manage again.  You will see two Vibration Settings, Intensity and Duration, that you may adjust between High, Medium, and Low to suit your needs.  As you adjust the settings, Keen will vibrate to provide an example of the setting.