It would be great if there is an option to run a cmd with the start / stop of xvfb. This way it would be possible e.g. to record a screencast of the xvfb contents using ffmpeg.

Example:
ffmpeg -f x11grab -vc x264 -s vga -r 30 -b 2000k -g 300 -i :$DISPLAY.0 $build_nr.avi