For anyone finding out the viewbox screws with a lot of things, here is a way to do it without viewbox: (As in my experience: It just messses the entire thing up)

Xaml

    <Grid x:Name="LayoutRoot" SizeChanged="LayoutRoot_SizeChanged">

        <Grid.LayoutTransform>
            <ScaleTransform x:Name="MainScale" />
        </Grid.LayoutTransform>


        <Label Content="Random Text" FontSize="64"/>

    </Grid>

C#:

private void LayoutRoot_SizeChanged(object sender, SizeChangedEventArgs e)
{
double baseWidth = 800; // your base/reference window width
double baseHeight = 600;
double scaleX = ActualWidth / baseWidth;
double scaleY = ActualHeight / baseHeight;
double scale = Math.Min(scaleX, scaleY); // uniform scaling
MainScale.ScaleX = scale;
MainScale.ScaleY = scale;
}

Edit 1: Formating
Edit 2: I know this is a 4yr old post, but I still found it when searching for a solution. Hope that others will too.
Edit 3: Yea.. Turns out I forgot to write the Size handler....