View 1 monitor in VNC session?

From the Manual, it seems, the option -clip would seem to do what you want:

-clip WxH+X+Y

   Only show the sub-region of the full display that corresponds to the rectangle
   geometry with size WxH and offset +X+Y. The VNC display has size WxH (i.e. smaller
   than the full display). This also works for -id/-sid mode where the offset is
   relative to the upper left corner of the selected window. An example use of this
   option would be to split a large (e.g. Xinerama) display into two parts to be
   accessed via separate viewers by running a separate x11vnc on each part.

   Use  '-clip  xinerama0'  to clip to the first xinerama sub-screen (if xinerama is
   active). xinerama1 for the 2nd sub-screen, etc. This way you don't need to figure
   out the WxH+X+Y of the desired xinerama sub-screen. screens are sorted in increasing
   distance from the (0,0) origin (I.e. not the Xserver's order).