[Vuejs]-How to add new css file from node module

As mentioned by @1sloc in comments, you are probably getting a 404 error page in HTML.

Till you find a better solution, you may do as follows: copy flexboxgrid.min.css to the static folder at the project root, and serve it as a static file from there. This is similar to how you would serve a favicon.ico for your app.

For details on handling static files, you may refer to the answer in this question:
How to set favicon.ico properly on vue.js webpack project?