So, you’re about to start an MVVM Windows Phone app, and you really want to be able to run unit tests against your view models using Visual Studio’s built-in test engine.
By default, you can’t do that for a phone app – you need to create a Phone Unit Test App that will wrap everything and spit out the test results when you run the app.
If you really want to be able to run tests within the IDE you can build your view models in a separate Portable Class Library (yes, those again). As long as you add .NET as a target framework for the VM library, you will be able to write your tests in the same way as you would for any other project type.
Of course, this approach is not appropriate for all functionality – for phone-specific functionality you will need to use the default (separate app) approach, or a mix of the two.