hi,
That is a PHP5.4 related thing -- previously the same situation was considered as PHP Notice and won't be logged, but with PHP5.4+ it is considered as PHP Warning.
The solution is pretty easy though.
Open this PHP file:
plugins/system/vm2_cart/vm2_cart.php
find the 61st line, which is this:
$data->products = array();
and replace it with this:
if (!isset($data)) $data = new stdClass();
$data->products = array();
then the PHP Warning will be gone.
Regards.
Support