WPF 4.0 MVVM Binding the UserControl DataContext from the MainWindow viewmodel

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WPF

user-controls

MVVM

Binding

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In order to enable drag-drop properly between two user controls, I need to call their viewmodels from the MainWindow viewmodel

I had thought that it would be as simple as this:

<UserControl x:Class="LookupTermWizard.View.Properties"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"
        xmlns:b="clr-namespace:LookupTermWizard.Behavior"
        xmlns:vm="clr-namespace:LookupTermWizard.ViewModel"
        Height="300" Width="500" removed="LightGray" DataContext="{Binding}" >

<Window x:Class="LookupTermWizard.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:vm="clr-namespace:LookupTermWizard.ViewModel"
        xmlns:v="clr-namespace:LookupTermWizard.View"
        xmlns:b="clr-namespace:LookupTermWizard.Behavior"
        Title="Lookup Term Wizard" Height="900" Width="1300">
    <Window.DataContext>
        <vm:MainWindowViewModel />
    </Window.DataContext>
        <StackPanel DockPanel.Dock="Left" VerticalAlignment="Top" Height="775" >
            <ContentControl>
                <v:Properties DataContext="{Binding PropertiesVM}" />
            </ContentControl>

And then instantiate the viewmodel for the user control from the MainWindowViewModel

PropertiesViewModel propertiesVM;

PropertiesViewModel PropertiesVM
{
    get { return propertiesVM; }
    set
    {
        propertiesVM = value;
        OnPropertyChanged("PropertiesVM");
    }
}
public MainWindowViewModel()
{
    PropertiesVM = new PropertiesViewModel();

}

Of course, it doesn't work that way. Any chance of getting some guidance?