I have the same problem and find the post:
http://forum.virtuemart.net/index.php?topic=121514.0
and also check VM 2.6.0 fixed code:
if (!class_exists ('shopFunctions')) require(JPATH_VM_SITE . DS . 'helpers' . DS . 'shopfunctions.php');
is "JPATH_VM_SITE" right?
the corrected one from the post is "JPATH_VM_ADMINISTRATOR"