[BRotondi]

O.K. Another solution with changed <div>: Put

    <div id="p-Spezial" class="generated-sidebar portlet">
        <a href="<?php echo htmlspecialchars($this->data['nav_urls']['mainpage']['href'])?>"><img src="<?php $this->text('logopath') ?>"></a>
    </div>

before

    <script type="<?php $this->text('jsmimetype') ?>"> if (window.isMSIE55) fixalpha(); </script>

in vbMediaWiki.php and then use FireBug for the customisations. This gives you a better starting point, since the original Code will put the logo over the vB-header.