Patch for the Drupal Varnish module (7.x-1.0-beta3 + https://www.drupal.org/files/issues/varnish_purge_limit-1481136-11_1.patch) to make it site alias aware.

varnish_site_alias.patch

diff --git a/varnish.cache.inc b/varnish.cache.inc
index 7628579..a18a846 100644
--- a/varnish.cache.inc
+++ b/varnish.cache.inc
@@ -71,10 +71,11 @@ class VarnishCache implements DrupalCacheInterface {
           varnish_purge_all_pages();
         }
         else {
-          $host = _varnish_get_host();
           $base = base_path();
           $purge = $cid . '(.*)';
-          varnish_purge($host, '^' . $base . $purge . '$');
+          foreach(_varnish_get_hosts() as $host) {
+            varnish_purge($host, '^'.$base.$purge.'$');
+          }
         }
       }
       elseif (is_array($cid)) {
diff --git a/varnish.module b/varnish.module
index f205629..cdc8638 100644
--- a/varnish.module
+++ b/varnish.module
@@ -141,8 +141,9 @@ function varnish_expire_cache($paths) {
     }
   }
   else {
-    $host = _varnish_get_host();
-    varnish_purge_paths($host, $paths);
+    foreach(_varnish_get_hosts() as $host) {
+      varnish_purge_paths($host, $paths);
+    }
   }
 }
 
@@ -151,8 +152,9 @@ function varnish_expire_cache($paths) {
  */
 function varnish_purge_all_pages() {
   $path = base_path();
-  $host = _varnish_get_host();
-  varnish_purge($host, $path);
+  foreach(_varnish_get_hosts() as $host) {
+    varnish_purge($host, $path);
+  }
 }
 
 /**
@@ -277,12 +279,42 @@ function theme_varnish_status($status) {
 }
 
 /**
- * Helper function to parse the host from the global $base_url
+ * Helper function to get the hosts (pseudonymous subdomains) we need to purge
+ * @return array of hosts
  */
-function _varnish_get_host() {
+function _varnish_get_hosts() {
   global $base_url;
+
+  // We need access to Drupal's site aliases. The file /sites/sites.php is included
+  // in drupal bootstrap, but unfortunately its contents are not stored anywhere.
+  if (file_exists(DRUPAL_ROOT . '/sites/sites.php')) {
+    include(DRUPAL_ROOT . '/sites/sites.php');
+  }
+
   $parts = parse_url($base_url);
-  return $parts['host'];
+  $currentHost = $parts['host'];
+  $hosts = array();
+
+  if (isset($sites) && count($sites)>0) {
+    // Find the other pseudonymous domains. We might be using an alias
+    // or the primary host, so find the primary host.
+    if (array_key_exists($currentHost, $sites)) {
+      // Request used an alias of the primary host
+      $primaryHost = $sites[$currentHost];
+    } else {
+      // we must be using the primary host.
+      $primaryHost = $currentHost;
+    }
+    // add pseudonyms
+    $hosts = array_merge($hosts, array_keys($sites, $primaryHost));
+    // add primary name
+    $hosts[] = $primaryHost;
+  }
+  if (count($hosts)==0) {
+    // ensure current host is added
+    $hosts[] = $currentHost;
+  }
+  return $hosts;
 }