Trigger Re-evaluation of Tab Stops

Question

I have an MVVM Light-based WPF application where I'm having problems setting focus on a particular button based on viewmodel conditions.

Here's the relevant XAML:

<ContentControl Grid.Column="0" Grid.Row="0" Grid.ColumnSpan="2"
                HorizontalAlignment="Stretch"
                VerticalAlignment="Stretch"
                IsTabStop="False"
                Content="{Binding PageControl, Mode=OneWay}"
                Name="ctlCurrent"/>

<Button Style="{DynamicResource MaoStandardButton}"
        IsDefault="True"
        support:FocusExtension.IsFocused="{Binding DefaultButtonIsSelected}"
        TabIndex="1">

    <Button.IsEnabled>
        <MultiBinding Converter="{StaticResource AllTrueConverter}">
            <Binding Path="IsDirty"/>
            <Binding Path="IsValid"/>
        </MultiBinding>
    </Button.IsEnabled>

    Save &amp; New
</Button>

There are two viewmodels in operation here. The first is the parent, to which the controls shown are bound. The second is bound to the UserControl, which is exposed by the parent's PageControl property (this second viewmodel is bound to the UserControl's DataContext).

The two viewmodels communicate behind the scenes, with the second/child viewmodel controlling the parent viewmodel properties of IsDirty and IsValid (this is done based on evaluating changes to the child viewmodel's properties, and their validity). Conceptually, what I want to happen is this: whenever the last control in the UserControl loses focus, and the child viewmodel IsDirty and IsValid, I want focus to shift to the Save & New button.

Part of implementing this involved adding an attached property to the Save & New button, based on this SO article.

That's the basis of the support:FocusExtension... XAML. The parent viewmodel property DefaultButtonIsSelected is set when the last control in the UserControl loses focus, and everything IsValid and IsDirty.

Unfortunately, while I can see the focus-controlling code in the attached property being called, as soon as the Save & New button gets focus, it immediately loses it (and the focus-lost code from that article gets called).

I'm pretty sure this is due to the timing of when WPF is evaluating the value of the button's IsEnabled property. I say this because commenting out the multiconverter -- so the button is always enabled -- lets the focus setting control work as expected -- focus is set on the button, and not lost.

Is there a way to force WPF to re-evaluate tab stops after the Save & New button's IsEnabled property changes? I think that might solve the problem. Other ideas are welcome, too, of course.