There's a vertical slider at the side of the vibrato box. When all the way to the top, you will not hear any vibrato at all unless you move your mod wheel. As you move that slider down, it will introduce vibrato, and the vibrato will begin to intensify the lower you move that slider. Basically, this sets the "neutral" amount of vibrato.
You will also notice a drop down box that probably says "wheel" on it right now, meaning if you move your mod wheel, the vibrato will intensify. With the side slider at the top and the mod wheel at the bottom, there will be no vibrato, but if you move your mod wheel while holding a note, you will gradually introduce vibrato into the sound.