Please ignore the above ... I closed Virtual DJ, disconnected the USB to my Numark V7s, Powered off the mixer and powered back on - reopened Virtual DJ, then selected the 14 MXR as the sound out device - then used simple midi wrapper in VDJ to map my fader.
Click on the + sign to add a device, moved the fader (which mapped to my fader to the key 2-CC31), then clicked on Action Learn button and selected the video x-fader (which gave me the value "video_crossfader")
All works well now!