vcard is a standard format for representing a contact, but doesn't necessarily need to be the storage format - like a one for one vcard text file per contact.

You could have the contacts in a sqlite database with any structure provided there was an interface that produced a vcard in response to a query.

Personally, I'd like to see that interface be groupdav provided it can be made efficient enough. That way we can make synchronisation a simple affair for tasks, contacts, calendar, etc.