I had the same problem with CMSMart's Ajax Drop Down Cart for Virtuemart 3.0.0.
Problem: Everything worked normally until the customer went to the cart. Then all of VirtueMart showed a white screen with only "Error" displayed.
Solution: Turning on debug, I got the same error: Call to undefined method VirtueMartCart::getCardCategoryId().
Disabling either the plugin or the module for CMSMart's Ajax Drop Down Cart for Virtuemart corrected the error, but prevented the drop down cart module from working.
The code that Jumbo! highlighted (mega thanks!) was not in the plugin code; it was in the module code. Location: /modules/mod_virtuemart_ajax_cart/helper.php. Commenting out the single line of code fixed the problem.
Note that there is a newer version 3.0.2, but I don't have access to it at the moment (paid component) to see if it solves this problem.