AO in Vue is also kinda tricky...  Unlike other programs (not Poser) it doesn't have any options other than size.  So you'll have to tweak the other options in other ways, otherwise you're looking at horrific render times for complex scenes with lots of Ecosystem instances and such.

I generally render out an AO pass with no textures, so it's just the light and dark values, then use that pass in composition.  Works really well, too!  It's both faster to render two images this way and also in Photoshop you can adjust the AO's strength afterwards as a separate layer...