Using ember-cli + ember-cli-sass to generate vendor.css

Hi guys. I’m a newbie using ember and I’m trying to use ember + ember-cli-sass but I can’t make it generate my vendor.css as I expected. With ember serve or ember build --env production, my app.css is generated successfully.

After read the docs about managing assets, I configure my ember-cli-build.js as it follow:

"use strict";

const EmberApp = require("ember-cli/lib/broccoli/ember-app");

module.exports = function(defaults) {
  let app = new EmberApp(defaults, {});


  return app.toTree();

What I’m doing wrong here?