Are you using a composite material or are you using a composite map in the diffuse slot of a standard material? The composite map has layers with blending options just like photoshop and should work if you put your AO layer above your base layer and set it to multiply or something similar.
What Frubes said is true but that is only if you want to render the Vertex paint, if you want to render with a diffuse AND vertex paint you just have to do a couple extra things. Use a composite Material, for your Base Material slot use a standard shader and put your diffuse texture in the Diffuse Slot. Going back to your…
Yeah Im pretty sure you guys are right, I'm running Max2008 and I don't get the cool options with the composite map on the diffuse :( So yeah, don't listen to me lol
you could do this a bit more cleanly with a composite texture. that way your not processing 2 materials. Just put a texture in 1 slot, create a new slot and put the vertex color in the top slot on multiply. this should render a little faster