vanilla/static isn't available until vanilla itself is.

pristine_app/config.ru

@@ -1,6 +1,7 @@
+require "application"
+
 # If you're running your site under a proper webserver, you probably don't need this.
 require 'vanilla/static'
 use Vanilla::Static, File.join(File.dirname(__FILE__), 'public')
 
-require "application"
 run Application.new