Is there any way to tell XFCE to "sense" a display even if xrandr tells it's disconnected?

After:
xrandr --addmode VGA1 1024x768
xrandr --output VGA1 --mode 1024x768 --right-of LVDS1
x11vnc -clip 1024x768+1280+0

I can view the VGA1 contents with a vnc viewer, but have the following problems:
- the panel extends through both "monitors" and there's no checkbox on the panel properties window to limit it to one
- popups and menus get displayed on LVDS1 to the right, despite their would be on VGA1
- windows cannot be fully moved to VGA1, some 16 pixels stay on LVDS1 if the window is moved to the right.
The Settings manager / Display window shows only the internal/LVDS1 monitor.
Plugging in a real monitor into VGA1 of course solves all the above.

Any help is appreciated.