In my view, I have:

<UserControl x:Class ... MouseDown="UserControl_MouseDown"> 
    <Viewport3D Name="Viewport" Grid.Column="0">
        ...
    </Viewport3D >
</UserControl>

In my code-behind, I have:

private void UserControl_MouseDown(object sender, MouseButtonEventArgs e)
{
    ((MapPanelViewModel)DataContext).OnMouseDown(e, Viewport);
}

And in my view-model, I have:

    public void OnMouseDown(MouseEventArgs e, Viewport3D viewport)
    {
        var range = new LineRange();
        var isValid = ViewportInfo.Point2DtoPoint3D(viewport, e.GetPosition(viewport), out range);
        if (!isValid)
            MouseCoordinates = "(no data)";
        else
        {
            var point3D = range.PointFromZ(0);
            var point = ViewportInfo.Point3DtoPoint2D(viewport, point3D);
            MouseCoordinates = e.GetPosition(viewport).ToString() + "\n" + point3D + "\n" + point;
        }
    }

I really don't have a good sense of how to handle mouse events with MVVM. I always just end up putting them in the code-behind and casting the DataContext as SomeViewModel, then passing the MouseEventArgs on to a handler in my view-model. That's bad enough already, but in this case, I'm actually passing in a control (a Viewport3D), which is necessary for translating coordinates between 2D and 3D.

Any suggestions on how to make this more in tune with MVVM?