The vibrate button is in the top left corner of the phone's border, just above the upper volume button.
When turned on, the vibrate button limits the sound projected from the phone so that only music can be heard. Calls and texts, when the button is activated, cause the phone to vibrate rather than play the ringtone/text tone. Sounds created by other apps cannot be heard while vibrate mode is activated.
To turn on vibrate mode, flick the button down. When this action is done, you will see orange above the button. Flick the button back upwards to deactivate vibrate mode. You should then see black underneath the button.
Note: this button is what you should flick to turn off the ringer for movies and plays