So theoretically it’s possible. You are however going to hit issues running Swift on an Arduino - I’m not sure there is any 4.1 toolchain for an Arduino, Raspberry Pi support still doesn’t really exist. If you can get Swift to build on an Arduino, you will be most of the way there.
If I’ve misunderstood the question and you just want to send POST requests from your Arduino without Vapor and send it to a Vapor server running in Vapor Cloud then, yep! That will definitely work! As for a tutorial, the Arduino essentially acts as any other client would, including RESTed. So if you can use your app from RESTed, then it will work on an Arduino