MVVM - pass info from MainWindow ViewModel to WinForms based child UserControl

Question

I'm working on a WPF/MVVM application that will host a WinForms graphical control for the display of CAD-type geometry.

First, a little background.

Basically, I have a MainWindow with a TreeView on the left and a TabControl on the right. The TreeView simply renders an Observable collection of TreeNode objects. Based on the type of node that's clicked in the tree, the main window's ViewModel creates an instance of an appropriate child ViewModel for the selected tree node (a "workspace"). The new workspace is added to another Observable collection that's bound to the TabControl - which creates a new tab item. This is very similar to the well-known Josh Smith example.

In the XAML of the main window, I'm using <DataTemplate DataType...>

to instantiate an appropriate View for the just created ViewModel.

So far, so good. That all works well. Now, here's where I'm stuck...

One "type" of tree node represents a CAD model. When a node of that type is selected, the view that's instantiated contains the WinForms graphical control (wrapped in a WinFormsHost UserControl). The underlying WinForms control has a "LoadFile()" method that simply requires a filename as input.

The object that's rendered as a tree node in the MainWindow contains the name of the file I need to load. So, I'm trying to figure out the best way to get the filename from the tree node object and pass it to the "LoadFile()" method in the underlying WinForms control.

I have the filename in the MainWindow's ViewModel at the time I create the ViewModel for the CAD control (which, in turn, creates the View containing the WinForms control via the XAML DataTemplate).

Every attempt I've made so far feels like I'm painting myself into a corner. So, am I already too far in the weeds, or does this sound salvageable?

Edited to post code as requested below...

Relevant code-behind of User Control

public string MSIFile
{
    get { return (String)GetValue(MSIFileProperty); }
    set { SetValue(MSIFileProperty, value); }
}

public static readonly DependencyProperty MSIFileProperty =
    DependencyProperty.Register("MSIFIle", typeof(string), typeof(ViewportUserControl),
    new FrameworkPropertyMetadata(null, new PropertyChangedCallback(OnMSIFileChanged)));


private static void OnMSIFileChanged(
   DependencyObject sender, DependencyPropertyChangedEventArgs e)
{
    Console.WriteLine(e);
}

User Control XAML

<UserControl x:Class="TruNest.UserControls.ViewportUserControl"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
             xmlns:vp="clr-namespace:CustomControls;assembly=Winforms"
             d:DesignHeight="300"
             d:DesignWidth="300"
             mc:Ignorable="d" Loaded="UserControl_Loaded">
    <Grid>
        <WindowsFormsHost HorizontalAlignment="Stretch" VerticalAlignment="Stretch">
            <vp:ViewportCustomUC x:Name="Wrapper" />
        </WindowsFormsHost>
    </Grid>
</UserControl>

Code from MainWindow ViewModel - triggered when new TreeNode is selected

else if (_selectedTreeViewItem is ViewportNode)
{
    ViewportNode node = _selectedTreeViewItem as ViewportNode;
    ViewportViewModel workspace = new ViewportViewModel();
    workspace.TreeNode = _selectedTreeViewItem;
    workspace.Name = String.Format("{0}", node.Name);
    Workspaces.Add(workspace);
    this.SetActiveWorkspace(workspace);
}