build/utils.js

utils.js

exports.cssLoaders = function(options) {
  ...
  ...
  function resolveResouce(fileName) {
    // Absolute Path
    return path.resolve(__dirname, '../src/assets/scss/' + fileName)
  }
  
  function generateSassResourceLoader() {
    var loaders = [
      cssLoader,
      'sass-loader', {
        loader: 'sass-resources-loader',
        options: {
          resources: [
            resolveResouce('mixins/index.scss'),
            resolveResouce('vars/index.scss')
          ]
        }
      }
    ];
    if (options.extract) {
      return ExtractTextPlugin.extract({
        use: loaders,
        fallback: 'vue-style-loader'
      })
    } else {
      return ['vue-style-loader'].concat(loaders)
    }
  }
  ...
  ...
}

Thanks mate works like a charm.