This is one of the features that bryce has that I wish Vue had.  I've often added a pile of pixels onto one side or the other of a bryce render when I've needed to re-aspect ratio it (and the new bit of render fits pixel perfectly onto the edge of your orginal camera view render - its the camera pan settings in the camera properties dialog for anyone thats interested), but as much as I've tried, the only way to expand the horizontal size of the vue render that I've found is to move the camera/change focal length and hence change the initial image. Vue's camera feels more real to me, but Bryce's strange pan property in that case was handy.

Dreams are just nightmares on prozac...
Digital WasteLanD