This thread solved the problem for me :
http://forum.virtuemart.net/index.php?topic=96541.0
Just go to your database (I'm using PHPMyAdmin) and go to your online store database, find table "XXX_virtuemart_userinfos" (XXX depends on how you set the name in the beginning, mostly using JOS" and change/edit the Admin's "virtuemart_user_id" from 42 to 1. It works for me, invoice mail sent to customer's email.