Closed For Maintenance 0.8.4
When you enable this plugin it will allow you to "close" your forum as you upgrade or make changes. You can customize closed.php to be whatever you want to be shown.
If you have any issues with redirect change line header to redirect to direct path ie: 'http://pathtovanilla/plugins/Maintenance/closed.php' ;
Now when in Maintenance mode, a bar will show to remind you to turn off the plugin. When you disable the plugin, visitors can access the site again.
Note: I recommend you use this plugin instead: https://vanillaforums.org/addon/1299/admin-mode instead.