You wrote, “Imagine working for a customer that is going to host their site on WPEngine and knowing with absolute confidence that the theme you created locally will work without issue.” Not possible to duplicate the WPEngine production environment with Vagrant. You may be able to come close, but not 100%. Why? Because WPEngine uses a highly proprietary method (they call it their secret sauce) to achieve super fast page loads. And unless you have some in with WPEngine and have access to this method, you can’t duplicate the production environment entirely. And there are issues that can come up in production that can not be duplicated outside of production. Not knocking WpEngine; I try to get all my clients on WPEngine.