I have a problem with my 'dual' monitor setup, with a *laptop being the main machine* and a VGA monitor attached to it. I need to switch the external monitor on and off as required.

Was very happy noticing I could easily have the external VGA on, with the 'dual monitor' setup with xrandr, by 'xrandr --output VGA1 --auto --above eDP1', but now I find I can't turn the VGA off. With

xrandr --output VGA1 --off

somehow the X goes berserk, scrambling and everything, and you can't even turn the X off, forcing a reboot.

I wanted to put some log messages here, but can't really find anything looking relevant.

Sorry for the vague way of raising a question but if there's anybody having a similar problem or who can think of a direction ---even where to find the log--- to resolve this please advise

Xrandr logs to /var/log/Xorg.0.log, you can check that for messages.

You could try using lxrandr to turn the screen on/off. It's a simple GUI around xrandr and it probably will yield the same result, but you can always try.