Hi,
Looks like you are using the old version of our VM Template which isn't fully supported by VM2.0.10. The new version doesn't have an issue such that. You might experience some other failures in other sections as well though.
But please try this patch for cart page.
Open this file:
ROOT\templates\zonnebron\html\com_virtuemart\cart\default.php
and look at the 66th line.
And change this line with this:
<?php echo shopFunctionsF::getLoginForm ($this->cart, FALSE); ?>
The problem "Undefined variable" should be gone.
Cheer.