Center header in menu control

How do I vertically center a header in the menu control?

This was my try:

<MenuItem Header="File" StaysOpenOnClick="True" FontFamily="Arial" VerticalAlignment="Center">
                 <MenuItem Header="Open" Click="Open_Click" IsEnabled="True"/>
             </MenuItem>
         </Menu>
 

But its aligned to the Top-left.

What am I doing wrong?

[EDIT]

My whole menu now looks like this:

<Menu Canvas.Left="0" Canvas.Top="0" Name="menu1" Margin="0,0,0,384">
         <MenuItem Header="File" StaysOpenOnClick="True" FontFamily="Arial" VerticalAlignment="Center">
             <MenuItem Click="Open_Click" IsEnabled="True">
                 <MenuItem.Header>
                     <TextBlock Text="Open" VerticalAlignment="Center"/>
                 </MenuItem.Header>
             </MenuItem>
         </MenuItem>
         </Menu>
 

The header text 'file' still isn't vertically centered (which is what i want to center).
What exactly is this code centering? Is it the text 'open'?

[/EDIT]

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