Changed alot of files, this is what happened.

I needed a functionality to change between UserControls, so what I did was making an empty interface IPageViewModel which every ViewModel besides CombinedViewModel is inheriting from, this makes it possible for me to create an collection of ViewModels in my CombinedViewModel.

I also added an  CurrentPageViewModel which I can set, that will notify the view which ViewModel is currently set.

In my View "MainWindow", I've added an Windows.Resource, this makes it possible for me to ask, which viewModel is active and based on that I can change the UserControl.
I also added an ContentControl which is listening to which viewModel is currently set.