Vanna doesn't include helpers so we have to include them in views separately to use them. The absence of this line was producing a deprecation warning for error_messages_for, because our helper redefines that method (it was removed from rails).

app/views/notifications/index.html.haml

@@ -1,6 +1,7 @@
 -self.extend AspectGlobalHelper
 -self.extend AspectsHelper
 -self.extend ApplicationHelper
+-self.extend ErrorMessagesHelper
 .span-13
   %h2
     %span.notification_count{:class => ('unread' if @notification_count > 0)}