Here you found the solution:
http://forum.virtuemart.net/index.php?topic=128962.15 post from Milbo:
The id is unique, but thank you very much for the hint, the real problem was the ID of the field, which used as fallback just the name and the new js has problems to handle [] in the id. Please try the attached file (renamed) at /administrator/components/com_virtuemart/helpers
Download the attached file in this post (you must log in)
grt
Baloe