Hi,
Looks like you are using an old Flexible Dropdown shopping plugin. This problem has been resolved while ago.
Here is the patch:
1- open this PHP file:
/plugins/system/FlexibleVm3cart/FlexibleVm3cart.php
2- Before this line:
VmConfig::loadJLang('com_virtuemart', true);
(Around 25th - 26th line, i can't be sure)
3- Insert this line:
if(!class_exists('VirtueMartCart')) require(JPATH_VM_SITE.'/helpers/cart.php');
So, if the Class:VirtueMartCart is not defined, the VM helper file gets loaded to define the Class:VirtueMartCart
Regards