Today by messing around with the RenderElements of Vray, I came to know about taking a render pass for AO!!
Yes, no more hassle like mental ray to separetly render the AO pass (or, is there any work around?)
Here is how :
1) Add VrayExtraTex in your RenderElements
2) Drag the empty 'texture' slot to your material editor
3) Add VRAYDIRT in diffuse slot, and give the settings according to your scene.
Voila, AO comes as pass, even with the lights and stuff ON.