Vertical Spacer in XUL

How do you add vertical spacers in XUL used in Firefox Addons? (Something similar to HTML <br /> or <html:br /> a vertical version of <spacer flex="1"/>)

Example:

<?xml version="1.0"?>

<!DOCTYPE overlay SYSTEM "chrome://theaddon/locale/main.dtd">

<vbox xmlns="http://ift.tt/ZJ3Hg8">

  <!-- want some vertical space here -->
  <setting title="&options.changelog.title;" type="control">
    <button label="&options.changelog.label;" 
      oncommand="openDialog('chrome://theaddon/content/changelog.xul', '',
      'dialog=no, modal=no, resizable=yes, width=500, height=600');"/>
  </setting>

</vbox>