Instead of a VisualBrush use a Control Template.
XAML
<Style x:Key="LockIcon"
TargetType="Path">
<Setter Property="Height"
Value="16" />
<Setter Property="Width"
Value="16" />
<Setter Property="Stretch"
Value="Uniform" />
<Setter Property="Data"
Value="F1 M 80.6667,55.1172L 80.6667,35.3333C 80.6667,15.851 64.815,3.05176e-005 45.3333,3.05176e-005C 25.8509,3.05176e-005 9.99998,15.851 9.99998,35.3333L 9.99998,55.1172L 0,56L 0,129.333L 45.3333,133.333L 90.6667,129.333L 90.6667,56L 80.6667,55.1172 Z M 20.6667,54.1771L 20.6667,35.3333C 20.6667,21.7318 31.7317,10.6667 45.3333,10.6667C 58.9349,10.6667 70,21.7318 70,35.3333L 70,54.1771L 45.3333,52L 20.6667,54.1771 Z " />
</Style>
<Style TargetType='TextBox'
x:Key='WithLockIcon'>
<Setter Property='MinHeight'
Value='26' />
<Setter Property="Padding"
Value="22,4" />
<Style.Triggers>
<Trigger Property="IsEnabled"
Value="false">
<!-- Use a control template instead of
Background property -->
<!--<Setter Property="Background"
Value="{StaticResource LockBrush}" />-->
<!--<Setter Property="Padding"
Value="22,4" />-->
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type TextBoxBase}">
<!-- Set background here!-->
<Border BorderThickness="4"
BorderBrush="Transparent"
HorizontalAlignment='Stretch'>
<StackPanel Background='#dddddd'
Orientation='Horizontal'>
<Path Style="{StaticResource LockIcon}"
Fill="Red" />
<ScrollViewer Margin="0"
x:Name="PART_ContentHost">
</ScrollViewer>
</StackPanel>
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
</Trigger>
</Style.Triggers>
</Style>
</Window.Resources>
<StackPanel>
<TextBox x:Name='textbox1'
Text='Example Text'
IsEnabled='False'
Style='{StaticResource WithLockIcon}' />
<ToggleButton Checked='ToggleButton_Checked'
Unchecked='ToggleButton_Unchecked'
Content='Change State' />
</StackPanel>