The problem is that the X screensaver is still enabled, as shown by typing
xset q on the command line:
prefer blanking: yes allow exposures: yes
timeout: 0 cycle: 600
The solution has been given in this thread.
xset s noblank -dpms on the command line.
But it is more convenient to create a
~/bin/disable_xsc.sh file containing
# disable the X screensaver
# The kde screensaver should still work, though
xset s noblank -dpms
Make sure that this script is executable
chmod u+x ~/bin/disable_xsc.sh
otherwise you would get a blinking "kwrite" in the taskbar, kde hanging, the taskbar responding very slowly, firefox would hang on startup, ...
if you forgot to chmod, just kill the offending kwrite process, and issue the chmod command.
Finally, going to the kde
configure desktop -> system administration -> startup and shutdown -> Autostart
in the "shell script path", type
~/bin/disable_xsc.sh, leave the "Create symlink" checked.
This script will be run on startup.
Note that the kde screensaver will still work correctly, when vlc is not running. Its configuration can be found in
configure desktop -> Hardware -> Display and monitor -> screen locker