WP7 - VirtualizingStackPanel bottom margin does not work - silverlight

Question

I have VirtualizingStackPanel in ListBox ItemsPanel. If I set Margin="0,0,0,50" then margin is not showed but if I set left margin (or top or right) Margin="50,0,0,0" then margin works correct.
If I change VirtualizingStackPanel for StackPanel then bottom margin works well.
This Is my code:
<Grid x:Name="ContentPanel"
Grid.Row="1"
Margin="12,0,12,0">
<ListBox x:Name="TasksListBox"
Margin="0"
ItemsSource="{Binding Tasks}"
HorizontalAlignment="Stretch"
ScrollViewer.VerticalScrollBarVisibility="Hidden">
<ListBox.ItemsPanel>
<ItemsPanelTemplate>
<!--<StackPanel Margin="0,0,0,14" />-->
<VirtualizingStackPanel Margin="0,0,0,100"
Height="Auto" />
</ItemsPanelTemplate>
</ListBox.ItemsPanel>
<ListBox.ItemTemplate>
<DataTemplate>
<Button Background="White"
Width="455"
Height="105"
Tag="{Binding Id}"
HorizontalContentAlignment="Stretch"
VerticalContentAlignment="Stretch"
Click="Button_Click_1">
<Grid HorizontalAlignment="Stretch"
VerticalAlignment="Stretch">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="Auto" />
<ColumnDefinition Width="Auto" />
<ColumnDefinition Width="Auto" />
<ColumnDefinition Width="*" />
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition Height="*" />
<RowDefinition Height="24" />
</Grid.RowDefinitions>
<TextBlock Grid.ColumnSpan="4"
Text="{Binding Name}"
FontSize="24"
Foreground="#400000" />
<Image Grid.Row="1"
Visibility="{Binding Project, Converter={StaticResource StringToVisibilityConverter}}"
Source="/Images/ProjectIcon.png"
Width="20"
Height="18"
HorizontalAlignment="Left"
VerticalAlignment="Top"
Margin="0,0,5,0" />
<TextBlock Grid.Row="1"
Grid.Column="1"
Visibility="{Binding Project, Converter={StaticResource StringToVisibilityConverter}}"
Text="{Binding Project, Converter={StaticResource ToUppercaseConverter}}"
FontSize="16"
Foreground="#666666"
HorizontalAlignment="Left"
VerticalAlignment="Top"
Margin="0,-2,9,0"
LineStackingStrategy="BlockLineHeight" />
<Image Grid.Row="1"
Grid.Column="2"
Visibility="{Binding Context, Converter={StaticResource StringToVisibilityConverter}}"
Source="/Images/ContextIcon.png"
Width="20"
Height="18"
HorizontalAlignment="Left"
VerticalAlignment="Top"
Margin="-3,1,1,0" />
<TextBlock Grid.Row="1"
Grid.Column="3"
Visibility="{Binding Context, Converter={StaticResource StringToVisibilityConverter}}"
Text="{Binding Context, Converter={StaticResource ToUppercaseConverter}}"
FontSize="16"
Foreground="#666666"
HorizontalAlignment="Left"
VerticalAlignment="Top"
Margin="0,-2,0,0"
LineStackingStrategy="BlockLineHeight" />
</Grid>
<toolkit:GestureService.GestureListener>
<toolkit:GestureListener Flick="GestureListener_Flick_1" />
</toolkit:GestureService.GestureListener>
<toolkit:ContextMenuService.ContextMenu>
<toolkit:ContextMenu IsZoomEnabled="False"
BorderBrush="#9c0605"
Foreground="#400000">
<toolkit:MenuItem Header="dokončeno"
Foreground="#400000"
Tag="{Binding Id}"
Click="MenuItem_Click_1" />
<toolkit:MenuItem Header="upravit"
Foreground="#400000"
Tag="{Binding Id}"
Click="MenuItem_Click_2" />
<toolkit:MenuItem Header="odstranit"
Foreground="#400000"
Tag="{Binding Id}"
Click="MenuItem_Click_3" />
</toolkit:ContextMenu>
</toolkit:ContextMenuService.ContextMenu>
</Button>
</DataTemplate>
</ListBox.ItemTemplate>
</ListBox>
</Grid>

Answer 1

Code Behind
ObservableCollection<string> data = new ObservableCollection<string>();
for (int i = 0; i < 40; i++)
{
data.Add("Item" + i);
}
lbTest.ItemsSource = data;
Xaml
<Grid x:Name="LayoutRoot"
Background="Transparent">
<ListBox x:Name="lbTest">
<ListBox.Style>
<Style TargetType="ListBox">
<Setter Property="Background"
Value="Transparent" />
<Setter Property="Foreground"
Value="{StaticResource PhoneForegroundBrush}" />
<Setter Property="ScrollViewer.HorizontalScrollBarVisibility"
Value="Disabled" />
<Setter Property="ScrollViewer.VerticalScrollBarVisibility"
Value="Auto" />
<Setter Property="BorderThickness"
Value="0" />
<Setter Property="BorderBrush"
Value="Transparent" />
<Setter Property="Padding"
Value="0,0,0,150" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="ListBox">
<ScrollViewer x:Name="ScrollViewer"
BorderBrush="{TemplateBinding BorderBrush}"
BorderThickness="{TemplateBinding BorderThickness}"
Background="{TemplateBinding Background}"
Foreground="{TemplateBinding Foreground}"
Padding="{TemplateBinding Padding}">
<ItemsPresenter />
</ScrollViewer>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</ListBox.Style>
</ListBox>
</Grid>