"Uncaught ReferenceError: Vue is not defined"

new Vue({
  el: '#app',
  data: {
    val: "x"
  },
...
shuffleDeck() {
  Vue.set(arg1, arg2, arg3) // A call to Vue.set happens here
 }
...

module.exports = {
  // ...
  shuffleDeck() {
    this.$set(arg1, arg2, arg3);
  }
  // ...
};