Vuejs HTTP post with flask backend

I am really perplexed by this behavior.

signUpForm() {
      this.$http.post('http://localhost:5000/signup_d4d', {user: this.user}).then(response => {

        // get body data
        this.success = response.body;
        alert('success');

      }, response => {
        // error callback
      });
    }

And the backend code:

@app.route('/signup_d4d', methods=['POST'])
@cross_origin()
def sign_up_d4d():
    form_data = json.loads(request.data)
    print(form_data)

    print('LOL')
    return jsonify({}), 200

The request doesn't even make it across to python backend.

signUpForm() {
      this.$http.post("http://localhost:5000/signup_d4d", {user: this.user},{headers: {'Accept': 'application/json'}});
      alert('Created!');
    }

This send the request across, but it results in the python flask framework throwing an error on the return:

127.0.0.1 - - [12/Nov/2017 14:54:26] "POST /signup_d4d HTTP/1.1" 500 -
Error on request:
Traceback (most recent call last):
  File "/Users/jayanthvenkatraj/git_repos/ihie/ihie_backend/ihie-resource-server/some/lib/python2.7/site-packages/werkzeug/serving.py", line 209, in run_wsgi
    execute(self.server.app)
  File "/Users/jayanthvenkatraj/git_repos/ihie/ihie_backend/ihie-resource-server/some/lib/python2.7/site-packages/werkzeug/serving.py", line 200, in execute
    write(data)
  File "/Users/jayanthvenkatraj/git_repos/ihie/ihie_backend/ihie-resource-server/some/lib/python2.7/site-packages/werkzeug/serving.py", line 181, in write
    self.wfile.write(data)
IOError: [Errno 32] Broken pipe

If anyone has a working sample of vuejs posting to flask backend, that will be super helpful