Product custom field plugin (vmextended) data table only allows for one row

[scisteffan]

Steps to reproduce:
* Add custom field, type = plugin, then select the pre-installed "Product Specification" custom field plugin.
* Go to product add/edit screen, add the newly added custom field. Enter "1" and "2" in the text value boxes.
* Add another custom field of the same type to the same product, enter "3" and "4" in the text value boxes.
* Click save

Rather than Saving the data independently for each custom field, both custom fields contain the same values in the text boxes.

I've looked at the PHP code, and the code in administrator/components/com_virtuemart/plugins/vmcustomplugin.php only supports one row in the data table for the plugin. There should be support for multiple rows, or the UI should prevent the user from adding multiple custom fields.

Using Safari 6.0.2 on OS X Mountain Lion, PHP 5.3.1.