gistfile1.yml

    templating:
        engines: ['twig']
        hinclude_default_template: 'Layouts/loading.html.twig'
        assets_base_urls:
          http: [ /symfony ]
          ssl:  [ /symfony ]

gistfile2.twig

        {% stylesheets 'vendor/semantic/semantic.css' filter='cssrewrite'%}
        <link href="{{ asset_url }}" type="text/css" rel="stylesheet" media="screen"/>
        {% endstylesheets %}

gistfile3.txt

GET http://XX.local/vendor/semantic/themes/default/assets/images/flags.png 404 (Not Found)

should be:

http://XX.local/symfony/vendor/semantic/themes/default/assets/images/flags.png