Since I am creating an animation the other way to do this is to render the color and alpha.  Then in Sony Vegas for editing I can create the mask by parenting the alpha to the color track. These are extra steps I would prefer not to do. I see not transparancy option in Esprit but in Poser it produces the image on a transparent background for you which is really really handy.

Windows 10 - Poser Pro 2012 - 64Bit - 24GB RAM - 4 x 3.40 GHZ processor