This is basically a link to a current category. So add this code to any file you wish (whatever part of the site you want it to appear on):

Virtuemart 1.1

$itemid = JRequest::getVar('Itemid', '');
$category_id = JRequest::getVar('category_id', '');

$url='/index.php?option=com_virtuemart&page=shop.browse&Itemid='. $itemid. '&category_id='. $category_id. '&limitstart=0';
echo '<a href="' . $url . '">Reset all filters</a>';
 

Virtuemart 2

$itemid = JRequest::getVar('Itemid', '');
$category_id = JRequest::getVar('virtuemart_category_id', '');

$url='/index.php?option=com_virtuemart&view=category&Itemid='. $itemid. '&virtuemart_category_id='. $category_id. '&limitstart=0';
echo '<a href="' . $url . '">Reset all filters</a>';