I need to provide a REST call from Plone to another service. Do I need to integrate something like Flask into Plone or can I do it without any other frameworks / libraries?
You can create your own REST API in plone using plone.rest:
https://pypi.python.org/pypi/plone.rest
plone.rest will most likely become part of the Plone kernel at some point.
If you want a ready-to-use off-the-shelf solution look at plone.restapi:
https://github.com/plone/plone.restapi
There is no release plone.restapi yet. Although, soon there will be one.