Hi Mat,
relatively simple
file templates/YOUR_TEMPLATE/html/com_virtuemart/productdetails/default.php ~ line 453
change
<li class="selected">
<a href="#" rel="desc"><span><?php echo JText::_('COM_VIRTUEMART_PRODUCT_DESC_TITLE') ?></span></a></li>
to
<?php if (!empty($this->product->product_desc)) { ?>
<li class="selected">
<a href="#" rel="desc"><span><?php echo JText::_('COM_VIRTUEMART_PRODUCT_DESC_TITLE') ?></span></a></li>
<?php } ?>