Plugin TEXT VARIABLE INPUT: Create a field in the product page with a variable passed from a previous page, which can be variable $ _GET or $ _POST (selectable option from the plugin).
(http://www.cyberspazio.eu/wp-content/uploads/2013/07/plugintextinputvariable.jpg)
Presentation / Instructions:
http://www.cyberspazio.eu/text-variable-input-plugin-virtuemart/
Download:
http://www.cyberspazio.org/file-da-scaricare/joomla/44-plugin-virtuemart-text-variable-input.html