Where are my Address Book vCards?

If your goal is to simply make a backup of all your contacts, you can select all your contacts, then use File>Export>Address Book Archive. It will create a folder with all the files necessary to rebuilt your contacts.

Although AddressBook.app works with vCards, it doesn't store them in that form. You can export all of your contacts into a single vCards.vcf file, or do them individually. But, I don't see an option, other than only doing one contact at a time, to have it export separate cards for each contact in a selected set.

If you want to do them all, and have separate vCard files for each contact, you can use Automator and setup a simple workflow using "Find Groups in Address Book" and "Export vCards" actions. In "Find Groups.." set up for the groups or lists of contacts you'd like to export. Then, in "Export..." you can specify whether you would like a single vCard for all, or individual vCards for each selected contact.