How to ignore plugin on Vue CLI3 in vue.congif.js

You appear to be trying to use the deprecated constructor. Try this instead and don't forget to import webpack into the script...

const webpack = require('webpack')

module.exports = {
  configureWebpack: {
    plugins: [
      new webpack.IgnorePlugin({
        resourceRegExp: /^\.\/locale$/,
        contextRegExp: /moment$/
      })
    ]
  }
}