Vray doesn’t really like doing AO any more - if you listen to their CG Garage podcast in the title song it actually says “Ambient Occlusion is passe”!
But you can still find the setting squirrelled away under the options in the Global Illumination settings (you have ensure that the settings are in Advanced Settings mode to see it. It is also only available as an option for a CPU render.
If you are after a separated AO pass however, you need to add an ‘Extra Texture’ render element to your stack - in the texture option for the element set the texture slot to a ‘Dirt’ texture, you then have plenty of control over Occlusion settings.
Hope this helps