Have you tried using Behavior… Here is a ScrollInViewBehavior. I have used it for ListView and DataGrid….. I thinks it should work for ListBox……

You have to add a reference to System.Windows.Interactivity to use Behavior<T> class

Behavior

public class ScrollIntoViewForListBox : Behavior<ListBox>
{
    /// <summary>
    ///  When Beahvior is attached
    /// </summary>
    protected override void OnAttached()
    {
        base.OnAttached();
        this.AssociatedObject.SelectionChanged += AssociatedObject_SelectionChanged;
    }

    /// <summary>
    /// On Selection Changed
    /// </summary>
    /// <param name="sender"></param>
    /// <param name="e"></param>
    void AssociatedObject_SelectionChanged(object sender,
                                           SelectionChangedEventArgs e)
    {
        if (sender is ListBox)
        {
            ListBox listBox = (sender as ListBox);
            if (listBox .SelectedItem != null)
            {
                listBox.Dispatcher.BeginInvoke(
                    (Action) (() =>
                                  {
                                      listBox.UpdateLayout();
                                      if (listBox.SelectedItem !=
                                          null)
                                          listBox.ScrollIntoView(
                                              listBox.SelectedItem);
                                  }));
            }
        }
    }
    /// <summary>
    /// When behavior is detached
    /// </summary>
    protected override void OnDetaching()
    {
        base.OnDetaching();
        this.AssociatedObject.SelectionChanged -=
            AssociatedObject_SelectionChanged;

    }
}

Usage

Add alias to XAML as xmlns:i="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity"

then in your Control

        <ListBox ItemsSource="{Binding Path=MyList}"
                  SelectedItem="{Binding Path=MyItem,
                                         Mode=TwoWay}"
                  SelectionMode="Single">
            <i:Interaction.Behaviors>
                <Behaviors:ScrollIntoViewForListBox />
            </i:Interaction.Behaviors>
        </ListBox>

Now When ever “MyItem” property is set in ViewModel the List will be scrolled when changes are refelected.