The ini files are not loaded properly in the index.php. If you move the ForceVirtualHost configuration to an extension the system does not take it, here is the code

     
eZSys::init( 'index.php', $ini->variable( 'SiteAccessSettings', 'ForceVirtualHost' ) == 'true' );

eZDebug::addTimingPoint( "Script start" );

$uri = eZURI::instance( eZSys::requestURI() );
$GLOBALS['eZRequestedURI'] = $uri;
require_once "pre_check.php";

// Check for extension
require_once( 'kernel/common/ezincludefunctions.php' );
eZExtension::activateExtensions( 'default' );

As can be seen in the code the ForceVirtualHost policy is loaded before the ini files for extensions so if you configure that policy in a extension it won't be loaded by the system

Steps to reproduce

Put the ForceVirtualHost within an extension and remove it from the site.ini.append preset on the override setting folder, it will load the default configuration.