In addition to Frank's answer about the incorrect use of url(), there may be a configuration issue. In a stock PHP installation, you have to make sure allow_url_fopen is enabled in your php.ini and allow_url_include is enabled in configuration or your script.
In my case, however, I'm running the Suhosin extension, which ignores these parameters and completely disables url_fopen by default. In order to include/require a vfsStream file, you need to add the vfs:// scheme to Suhosin's whitelist in php.ini: suhosin.executor.include.whitelist = "vfs://"
Thanks to Frank Kleine, the vfsStream maintainer, for helping me track this down.1