You should be able to use their official client libraries in your app engine project like any other API. Just make sure to enable the specific API for your specific project in the API Manager [0].
There's also a repo with sample code for various languages although not app engine specific [1]
Accessing Google APIs from Python
For your App Engine project, run this to install the client library first. This is considered a third party library, so you will need some initial setup for your app engine project as described here [2] -
$ mkdir lib
$ pip install --upgrade -t lib google-api-python-client
Create a file appengine_config.py
with the following content -
# appengine_config.py
from google.appengine.ext import vendor
# Add any libraries install in the "lib" folder.
vendor.add('lib')
You can find more information about the client library here -
https://developers.google.com/api-client-library/python/
Feel free to update your question with more specific issues you are having including some sample code you've tried and I'd be happy to provide more assistance.
[0] https://console.cloud.google.com/apis/api/vision.googleapis.com/overview
[1] https://github.com/GoogleCloudPlatform/cloud-vision
[2] https://cloud.google.com/appengine/docs/standard/python/tools/using-libraries-python-27#vendoring