These steps worked for me in Ubuntu 12.04 LTS x86_64:
You need to create a new site file at /etc/apache2/sites-available/john.mysite.com.conf
The extension ".conf" is very important, without it, the rest won't work!!
Then set up the site something like this:
Allow from all
Save the changes and enable the site with the command:
sudo a2ensite john.mysite.com
Reload apache to enable the site with:
sudo service apache2 reload
Restart apache service:
sudo service apache2 restart
After that, you need to set up DNS to point to the site as well (as a Subdomain), DNS will take some time but in my experience it only takes a few minutes (15).
Hope this helps!! :)