If you need an additional field for your own business requirement, you can do that by changing data model as well as some php code.
To add new field for product you have to make changes in php code of 3 or more separate files in the previous Virtuemart joomla 1.5 but in virtuemart 2.0 and joomla 1.7 version you only have to make change in only one file to add field.
To add field in virtuemart admin side, first you have to create field in your product table into database then you need to add field for product in virtuemart.
Follow this path from your site root
File Name: product_edit_information.php for product
Add this type of code wherever you want to put new field.
<td width="21%" >[Your_Field_Name] </td> <td > <input type="text" class="inputbox" name="Your_Field_Name" value="<?php echo $this->product-> Your_Field_Name; ?>" size="32" maxlength="64" /> </td>
I hope this will help you.