Address book does not store vcards. It can import them and export them, but does not store its data in vcard format. It stores one master database, and then a group of individual files. The master database is in the Applicaiton Support folder (I think - not 100% sure and I'm at work so I can't check). This is the master database for your address book data..
But the series of small files - these are what spotlight indexes. All they basically do is provide some basic data and point to a specific entry inside the address book database. When you search spotlight, it finds these individual files. When you open one, Address Book opens and finds your entry in its main database.
So... long story short: Address book doesn't store vcards.
-blake