I haven't done it, but if you look in vizard help for Generic Capture Devices you'll find:
"This VideoCamera implementation provides an interface to connect to any standard DirectShow capture device. This commonly includes web cameras and DV cameras."
You might be able to use some video capture software like VLC to capture the desktop and stream it out via directshow.
http://www.videolan.org/vlc/streaming.html
You can open your screen from vlc by opening "screen://"