I'm just installing the promise-polyfill as part of my package.json dependencies:

"promise-polyfill": "^6.0.2"

And then in my index.js Webpack entry, I'm importing it along with all my other imports:

import './promise-polyfill'
import jquery from 'jquery'
import tether from 'tether'
import bootstrap from 'bootstrap'
import './scss/base.scss';

import { app } from './app'
app.$mount('#app')

app.js, btw, contains my regular Vue imports:

import Vue from 'vue'
import { sync } from 'vuex-router-sync'
import App from './components/App'
import router from './router'
import store from './store'
sync(store, router)

import Meta from 'vue-meta'
Vue.use(Meta)

const app = new Vue({
  router,
  store,
  Meta,
  ...App
})

export { app, Meta, router, store }