extract-text-webpack-plugin is now deprecated for Webpack 4 - see home page: https://github.com/webpack-contrib/extract-text-webpack-plugin :

"Since webpack v4 the extract-text-webpack-plugin should not be used for css. Use mini-css-extract-plugin instead."

Are you planning on upgrading to the mini-css-extract-plugin?

If not, any way you could show the changes to make to the webpack.vendor.config.js ? (We're struggling with and I suspect many others are)

Thanks for any input :)