Vue does have a problem in some volumetric situations with alpha planes. I know it is being worked on, but I have run into the same thing many times. I just don't use volumetric on anything that may be an alpha plane basically until there is a solution. Alpha Planes, Poser, Xfrog trees, have all given me problems in volumetric settings. So don't feel alone by any means. :O) I could be wrong but I doubt it perhaps enclude a picture of what you are working on to make sure it is the same problem if you like. Take care.