$this-> refers to the CURRENT object. AS IN whats in the URL. SO, when you are on the category page , and you use $this-> ITS ONLY going to call information about the current category.

$this->   ON THE product page, calls product variables,

so, on the category page, calling product variables is like this
$product->

Here is what You want
<?php echo $product->product_sku ?>

THEN:
read these two threads, or atleast use them next time
http://forum.virtuemart.net/index.php?topic=97744.0
http://forum.virtuemart.net/index.php?topic=92756.0