I recently started to build Vue SPAs with ProcessWire as the backend, connected with a REST API. Thanks to code and the help of @LostKobrakai (How to use FastRoute with ProcessWire) and @clsource (REST-Helper) I got it up and running pretty quickly and now have put all of it in a site profile for others to use. It includes the REST API with routing for different endpoints, JWT Auth and a simple Vue SPA which shows the process of logging in a user (nevertheless, you don't have to use the Vue part, the API will work on it's own).
Check it out here: https://github.com/thomasaull/RestApiProfile
I'm pretty sure, it's not the perfect or most sophisticsted solution, but it gets the job done for me…
Feedback or Improvements are very welcome
Update: This site profile is a module now: https://github.com/thomasaull/RestApi