Binding a ViewModel to a view problem

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WPF

MVVM

I am new to MVVM so exuse me if this is very simple question

i have a listbox that display a list of Categories types for products when the user select any category type, a list of total expenses for each category of the selected category type is displayed as a view into an expander:

<Expander Header="{Binding ElementName=cattypeslistbox,Path=SelectedItem.Cat_Type}"
                      Visibility="{Binding Path=ExpanderIsVisible,Mode=TwoWay}"
                      IsExpanded="{Binding Path=ExpanderIsExpanded,Mode= TwoWay}" Height="167" Name="expander1" Width="357" >
                <Border Background="GhostWhite" BorderBrush="LightGray" BorderThickness="1" CornerRadius="5">
                    <ItemsControl ItemsSource="{Binding CurrentViewModel,Mode=TwoWay}" Margin="4" />
                </Border>
            </Expander>

in my main viewmodel i have currentviewmodel as viewmodelbase:

VMBase _currentViewModel;
public VMBase CurrentViewModel
{
   get
   {
       return _currentViewModel;
   }
   set
   {
       _currentViewModel = value;
       OnPropertyChanged("CurrentViewModel");
   }
}

public Category_Types SelectedCategoryTypes
{
   get
   {
       return _selectedCategoryTypes;
   }
   set
   {
       _selectedCategoryTypes = value;
       OnPropertyChanged("SelectedCategoryTypes");
       ShowCatsTotalExpenses();
   }
}

void ShowCatsTotalExpenses()
{         
    CatsTotalExpensesVM vm = new CatsTotalExpensesVM(_categoryTypesRepository, _selectedCategoryTypes, _from, _to);

    _currentViewModel = vm;
    ExpanderIsVisible = Visibility.Visible;
    ExpanderIsExpanded = true;
}

the problem is that this doesn't work nothing is displayed in the expander;

but when define a list of VieModels as following:

ObservableCollection<VMBase> _viewModels;
public ObservableCollection<VMBase> ViewModels
        {
            get
            {
                if (_viewModels == null)
                    _viewModels = new ObservableCollection<VMBase>();
                return _viewModels;
            }
        }

and in the ShowCatsTotalExpenses()

CatsTotalExpensesVM vm = new CatsTotalExpensesVM(_categoryTypesRepository, _selectedCategoryTypes, _from, _to);
_viewModels.Clear();
_viewModels.Add(vm);

then when binding to thses list of viewmodels it works fine and display the view:

<ItemsControl ItemsSource="{Binding ViewModels,Mode=TwoWay}" Margin="4" />

So what is the problem with the first CurrentViewModel how to get it work and what i am missing.

Thanks;