The twitter AddThis share used to shorten the URL before Tweeting now it just puts the full URL in. Why?

Hi,

How would I know? This a decision of "AddThis.com".
We have used their "share buttons" service. If you would like; you can try sharethis.com (or google it and find another service). Basically all of them work the same way. Once you generate your HTML from sharethis.com (or any other service) copy-paste them to product details page. The product details page you need to make edits is here:

ROOT/templates/<yourjoomlatemlatefolder>/html/com_virtuemart/productdetails/default.php

Cheers.