[Vuejs]-Convert anonymous user with Firebase: Cannot read property 'getAuthResponse' of undefined

You can just call linkWithRedirect on the anonymous user.

// Assuming you previously signed in anonymously.
firebase.auth().signInAnonymously().then(function(result) {
  var provider = new firebase.auth.GoogleAuthProvider();
  // Page will redirect to google and then back. You can get back
  // result via getRedirectResult().
  return firebase.auth().currentUser.linkWithRedirect(provider);
}).catch(function(error) {
  // Error occurred.
});