diff --git a/flask/helpers.py b/flask/helpers.py
index 2bcdc10b..f37be677 100644
--- a/flask/helpers.py
+++ b/flask/helpers.py
@@ -1004,17 +1004,3 @@ def is_ip(value):
             return True
 
     return False
-
-
-def patch_vary_header(response, value):
-    """Add a value to the ``Vary`` header if it is not already present."""
-
-    header = response.headers.get('Vary', '')
-    headers = [h for h in (h.strip() for h in header.split(',')) if h]
-    lower_value = value.lower()
-
-    if not any(h.lower() == lower_value for h in headers):
-        headers.append(value)
-
-    updated_header = ', '.join(headers)
-    response.headers['Vary'] = updated_header
diff --git a/flask/sessions.py b/flask/sessions.py
index a4b8cad1..3a5e2ac3 100644
--- a/flask/sessions.py
+++ b/flask/sessions.py
@@ -19,7 +19,6 @@ from itsdangerous import BadSignature, URLSafeTimedSerializer
 from werkzeug.datastructures import CallbackDict
 from werkzeug.http import http_date, parse_date
 
-from flask.helpers import patch_vary_header
 from . import Markup, json
 from ._compat import iteritems, text_type
 from .helpers import is_ip, total_seconds
@@ -407,7 +406,7 @@ class SecureCookieSessionInterface(SessionInterface):
 
         # Add a "Vary: Cookie" header if the session was accessed at all.
         if session.accessed:
-            patch_vary_header(response, 'Cookie')
+            response.vary.add('Cookie')
 
         if not self.should_set_cookie(app, session):
             return
diff --git a/tests/test_basic.py b/tests/test_basic.py
index e4d6b2f9..895f4f30 100644
--- a/tests/test_basic.py
+++ b/tests/test_basic.py
@@ -9,20 +9,21 @@
     :license: BSD, see LICENSE for more details.
 """
 
-import pytest
-
-import re
-import uuid
-import time
-import flask
 import pickle
+import re
+import time
+import uuid
 from datetime import datetime
 from threading import Thread
-from flask._compat import text_type
-from werkzeug.exceptions import BadRequest, NotFound, Forbidden
+
+import pytest
+import werkzeug.serving
+from werkzeug.exceptions import BadRequest, Forbidden, NotFound
 from werkzeug.http import parse_date
 from werkzeug.routing import BuildError
-import werkzeug.serving
+
+import flask
+from flask._compat import text_type
 
 
 def test_options_work(app, client):
@@ -523,14 +524,14 @@ def test_session_vary_cookie(app, client):
     @app.route('/vary-cookie-header-set')
     def vary_cookie_header_set():
         response = flask.Response()
-        response.headers['Vary'] = 'Cookie'
+        response.vary.add('Cookie')
         flask.session['test'] = 'test'
         return response
 
     @app.route('/vary-header-set')
     def vary_header_set():
         response = flask.Response()
-        response.headers['Vary'] = 'Accept-Encoding, Accept-Language'
+        response.vary.update(('Accept-Encoding', 'Accept-Language'))
         flask.session['test'] = 'test'
         return response