diff --git a/wp-admin/network.php b/wp-admin/network.php
index 4caf4cdea..351f7432f 100644
--- a/wp-admin/network.php
+++ b/wp-admin/network.php
@@ -40,7 +40,7 @@ function network_domain_check() {
  * Allow subdomain install
  *
  * @since 3.0.0
- * @return bool - whether subdomain install is allowed
+ * @return bool Whether subdomain install is allowed
  */
 function allow_subdomain_install() {
 	$domain = preg_replace( '|https?://[^/]|', '', get_option( 'siteurl' ) );
@@ -237,14 +237,14 @@ function network_step2( $errors = false ) {
 		echo '<div class="error">' . $errors->get_error_message() . '</div>';
 
 	if ( $_POST ) {
-		$vhost = !allow_subdomain_install() ? false : (bool) $_POST['subdomain_install'];
+		$subdomain_install = allow_subdomain_install() ? ! empty( $_POST['subdomain_install'] ) : false;
 	} else {
 		if ( is_multisite() ) {
-			$vhost = is_subdomain_install();
+			$subdomain_install = is_subdomain_install();
 ?>
 	<div class="updated"><p><strong><?php _e( 'Notice: The Network feature is already enabled.' ); ?></strong> <?php _e( 'The original configuration steps are shown here for reference.' ); ?></p></div>
 <?php	} else {
-			$vhost = (bool) $wpdb->get_var( "SELECT meta_value FROM $wpdb->sitemeta WHERE site_id = 1 AND meta_key = 'subdomain_install'" );
+			$subdomain_install = (bool) $wpdb->get_var( "SELECT meta_value FROM $wpdb->sitemeta WHERE site_id = 1 AND meta_key = 'subdomain_install'" );
 ?>
 	<div class="error"><p><strong><?php _e('Warning:'); ?></strong> <?php _e( 'An existing WordPress network was detected.' ); ?></p></div>
 	<p><?php _e( 'Please complete the configuration steps. To create a new network, you will need to empty or remove the network database tables.' ); ?></p>
@@ -265,7 +265,7 @@ function network_step2( $errors = false ) {
 			<li><p><?php printf( __( 'Add the following to your <code>wp-config.php</code> file in <code>%s</code> <strong>above</strong> the line reading <code>/* That&#8217;s all, stop editing! Happy blogging. */</code>:' ), ABSPATH ); ?></p>
 				<textarea class="code" readonly="readonly" cols="100" rows="7">
 define( 'MULTISITE', true );
-define( 'VHOST', '<?php echo $vhost ? 'yes' : 'no'; ?>' );
+define( 'SUBDOMAIN_INSTALL', <?php echo $subdomain_install ? 'true' : 'false'; ?> );
 $base = '<?php echo $base; ?>';
 define( 'DOMAIN_CURRENT_SITE', '<?php echo $hostname; ?>' );
 define( 'PATH_CURRENT_SITE', '<?php echo $base; ?>' );
@@ -309,9 +309,9 @@ RewriteBase ' . $base . '
 RewriteRule ^index\.php$ - [L]
 
 # uploaded files
-RewriteRule ^' . ( $vhost ? '' : '([_0-9a-zA-Z-]+/)?' ) . 'files/(.+) wp-includes/ms-files.php?file=$' . ( $vhost ? 1 : 2 ) . ' [L]' . "\n";
+RewriteRule ^' . ( $subdomain_install ? '' : '([_0-9a-zA-Z-]+/)?' ) . 'files/(.+) wp-includes/ms-files.php?file=$' . ( $subdomain_install ? 1 : 2 ) . ' [L]' . "\n";
 
-if ( ! $vhost )
+if ( ! $subdomain_install )
 	$htaccess_file .= "\n# add a trailing slash to /wp-admin\n" . 'RewriteRule ^([_0-9a-zA-Z-]+/)?wp-admin$ $1wp-admin/ [R=301,L]' . "\n";
 
 $htaccess_file .= "\n" . 'RewriteCond %{REQUEST_FILENAME} -f [OR]
@@ -319,7 +319,7 @@ RewriteCond %{REQUEST_FILENAME} -d
 RewriteRule ^ - [L]';
 
 // @todo custom content dir.
-if ( ! $vhost )
+if ( ! $subdomain_install )
 	$htaccess_file .= "\n" . 'RewriteRule  ^([_0-9a-zA-Z-]+/)?(wp-(content|admin|includes).*) $2 [L]
 RewriteRule  ^([_0-9a-zA-Z-]+/)?(.*\.php)$ $2 [L]';
 
@@ -327,7 +327,7 @@ $htaccess_file .= "\nRewriteRule . index.php [L]";
 
 ?>
 			<li><p><?php printf( __( 'Add the following to your <code>.htaccess</code> file in <code>%s</code>, replacing other WordPress rules:' ), ABSPATH ); ?></p>
-				<textarea class="code" readonly="readonly" cols="100" rows="<?php echo $vhost ? 11 : 16; ?>">
+				<textarea class="code" readonly="readonly" cols="100" rows="<?php echo $subdomain_install ? 11 : 16; ?>">
 <?php echo wp_htmledit_pre( $htaccess_file ); ?>
 </textarea></li>
 		</ol>
diff --git a/wp-includes/ms-default-constants.php b/wp-includes/ms-default-constants.php
index 23e04f3c4..38dce50fb 100644
--- a/wp-includes/ms-default-constants.php
+++ b/wp-includes/ms-default-constants.php
@@ -91,4 +91,50 @@ function ms_file_constants(  ) {
 	if ( !defined( 'WPMU_ACCEL_REDIRECT' ) )
 		define( 'WPMU_ACCEL_REDIRECT', false );
 }
+
+/**
+ * Defines Multisite subdomain constants and handles warnings and notices.
+ *
+ * VHOST is deprecated in favor of SUBDOMAIN_INSTALL, which is a bool.
+ *
+ * On first call, the constants are checked and defined. On second call,
+ * we will have translations loaded and can trigger warnings easily.
+ *
+ * @since 3.0.0
+ */
+function ms_subdomain_constants() {
+	static $error = null;
+	static $error_warn = false;
+	
+	if ( false === $error )
+		return;
+
+	if ( $error ) {
+		$vhost_deprecated = __( 'The constant <code>VHOST</code> <strong>is deprecated</strong>. Use the boolean constant <code>SUBDOMAIN_INSTALL</code> in wp-config.php to enable a subdomain configuration. Use is_subdomain_install() to check whether a subdomain configuration is enabled.' );
+		if ( $error_warn ) {
+			trigger_error( __( '<strong>Conflicting values for the constants VHOST and SUBDOMAIN_INSTALL.</strong> The value of SUBDOMAIN_INSTALL will be assumed to be your subdomain configuration setting.' ) . ' ' . $vhost_deprecated, E_USER_WARNING );
+		} else {
+	 		_deprecated_argument( 'define()', '3.0', $vhost_deprecated );
+		}
+		return;
+	}
+
+	if ( defined( 'SUBDOMAIN_INSTALL' ) && defined( 'VHOST' ) ) {
+		if ( SUBDOMAIN_INSTALL == ( 'yes' == VHOST ) ) {
+			$error = true;
+		} else {
+			$error = $error_warn = true;
+		}
+	} elseif ( defined( 'SUBDOMAIN_INSTALL' ) ) {
+		define( 'VHOST', SUBDOMAIN_INSTALL ? 'yes' : 'no' );
+	} elseif ( defined( 'VHOST' ) ) {
+		$error = true;
+		define( 'SUBDOMAIN_INSTALL', 'yes' == VHOST );
+	} else {	
+		define( 'SUBDOMAIN_INSTALL', false );
+		define( 'VHOST', 'no' );
+	}
+}
+add_action( 'init', 'ms_subdomain_constants' );
+
 ?>
diff --git a/wp-includes/ms-load.php b/wp-includes/ms-load.php
index ecb83d77a..bcb075cda 100644
--- a/wp-includes/ms-load.php
+++ b/wp-includes/ms-load.php
@@ -16,6 +16,9 @@
  * @return bool True if subdomain configuration is enabled, false otherwise.
  */
 function is_subdomain_install() {
+	if ( defined('SUBDOMAIN_INSTALL') )
+		return SUBDOMAIN_INSTALL;
+
 	if ( defined('VHOST') && VHOST == 'yes' )
 		return true;
 
diff --git a/wp-includes/ms-settings.php b/wp-includes/ms-settings.php
index cc0691cad..14862f9e1 100644
--- a/wp-includes/ms-settings.php
+++ b/wp-includes/ms-settings.php
@@ -21,6 +21,9 @@ require( ABSPATH . WPINC . '/ms-default-constants.php' );
 if ( defined( 'SUNRISE' ) )
 	include_once( WP_CONTENT_DIR . '/sunrise.php' );
 
+/** Check for and define SUBDOMAIN_INSTALL and the deprecated VHOST constant. */
+ms_subdomain_constants();
+
 if ( !isset( $current_site ) || !isset( $current_blog ) ) {
 
 	$domain = addslashes( $_SERVER['HTTP_HOST'] );
diff --git a/wp-includes/wp-db.php b/wp-includes/wp-db.php
index 64e812be1..de63b7188 100644
--- a/wp-includes/wp-db.php
+++ b/wp-includes/wp-db.php
@@ -575,7 +575,7 @@ class wpdb {
 			foreach ( $this->tables( 'global' ) as $table => $prefixed_table )
 				$this->$table = $prefixed_table;
 
-			if ( defined( 'VHOST' ) && empty( $this->blogid ) )
+			if ( is_multisite() && empty( $this->blogid ) )
 				return $old_prefix;
 
 			$this->prefix = $this->get_blog_prefix();