There are already posts about this - have you used the search?
basically depending on what view your using in templates/your_template/html/com_virtuemart/category/
e.g. default.php you need to add the code something like below - you will need to experiment as this template doesn't offer manufacturers links in the cat view
<?php
$ManufacturerProd = JRoute::_('index.php?option=com_virtuemart&view=category&virtuemart_manufacturer_id=' . $this->product->virtuemart_manufacturer_id);
$ManufacturerLink = JRoute::_('index.php?option=com_virtuemart&view=manufacturer&virtuemart_manufacturer_id=' . $this->product->virtuemart_manufacturer_id . '&tmpl=component');
$ManufacturerName = $this->product->mf_name;
?>
<div class="manufacturerlink">
See other products from
<a title="<?php echo $ManufacturerName; ?>" href="<?php echo $ManufacturerProd ?>">
<?php echo $ManufacturerName; ?>
</a>
</div>