WhenAnyValue is a method on ReactiveObject that can turn INotifyPropertyChanged.PropertyChanged events into an IObservable.

var layerNotes = this
      .WhenAnyValue(vm => vm.LayerViewModel)
      .Select(vm => vm.Notes)
      .ToProperty(this, vm => vm.Notes);

The code above will actually only push property changes through when the LayerViewModel property is changed. Not when its Notes property changes.

The correct way is to use the complete property chain in the call to WhenAnyValue like the following code:

var layerNotes = this
      .WhenAnyValue(vm => vm.LayerViewModel.Notes)          
      .ToProperty(this, vm => vm.Notes);

The code above will push property changes containing the content of the LayerViewModel.Notes every time the LayerViewModel property changes or its Notes property changes.