I do a simlar thing with webmin.
I run the following via cron every 5 mins:
[ -z "`ps -A | grep miniserv.pl`" ] && /etc/webmin/start
You will need to replace miniserv.pl with whatever the process is called and /etc/webmin/start with the command to start x11vnc.
However, I suspect there is an option so that x11vnc does not end when the last client disconnects - I don't actually know x11vnc.
Use both and you'll cover about as many situations as you can.