Where is "vue.config.js" file?

It's simple just create file vue.config.js in ROOT folder of project.

Its very important file. it's on top of vue project. People usualy use more than one page in old fasion style. vue.config.js is right place to define main dependency pages.

I used to create main sigle-page app (pwa) but i also need some other pages. Like error pages or google verify for example.

You can change dev server port , sourceMap enable/disable or PWA configuration...

module.exports = {
  pages: {
    'index': {
      entry: './src/main.ts',
      template: 'public/index.html',
      title: 'Welcome to my vue generator project',
      chunks: ['chunk-vendors', 'chunk-common', 'index']
    },
    'bad': {
      entry: './src/error-instance.ts',
      template: 'public/bad.html',
      title: 'Error page',
      chunks: ['chunk-vendors', 'chunk-common', 'index']
    },
    /* Disabled - Only one time
    'googleVerify': {
      entry: './src/error-instance.ts',
      template: 'public/somelink.html',
      title: 'Error page',
      chunks: ['chunk-vendors', 'chunk-common', 'index']
    },
    */

  },
  devServer: {
    'port': 3000
  },
  css: {
    sourceMap: false
  },
  pwa: {
    name: 'My App',
    themeColor: '#4DBA87',
    msTileColor: '#000000',
    appleMobileWebAppCapable: 'yes',
    appleMobileWebAppStatusBarStyle: 'black',
  },
}

For this config here's main instance file:

import Vue from 'vue'
import App from './App.vue'
import store from './store'

Vue.config.productionTip = false

new Vue({
  store,
  render: h => h(App, {
     props: { error: 'You can not search for assets...' }
  }),
}).$mount('#error')

See the documentation of Vue CLI:

vue.config.js is an optional config file that will be automatically loaded by @vue/cli-service if it's present in your project root (next to package.json). You can also use the vue field in package.json, but do note in that case you will be limited to JSON-compatible values only.

The file should export an object containing options:

// vue.config.js
module.exports = {
    // options...
}

So just create the file by yourself. It is completely optional.