Changed all UC_usercontrols, I couldn not use Datacontext because that would give me an error when trying to change the usercontrol, something with changing to a new usercontrol it would try to find the binding names in the old ViewModel even though a new was put into the datacontext, instead I'm manually binding everything now.

I made a dictionary (string, IPageViewModel) - To have a overview of all ViewModels and easy access with key value.

In all ViewModel's I added event when the given ViewModel want to change UserControl, example when I click the login button it should change the view to UC_LoggingIn, so the ViewModel is firing off the event which CombinedViewModel is listening to.
Each ViewModel in its constructor is given a reference to the dictionary of all ViewModels, so when the event is fired it simply tells CombinedViewModel in the event which ViewModel it should change the view to.

You might wonder how telling which ViewModel automatically changes view?
In MainWindow.xaml I have a Window.Resources which see's which ViewModel is active and based on that it changes to the given View.