First of all I dont agree with the Virtual Directory definition given. I don't agree that virtual directory needs to know all the information about identities, that's just how one of the virtual directory available in market, in IMHO that's crudest form of virtual directory. Most of the virtual directories just know the location of identity information for each virtualization zone/node that's it! It's a full blown directory, just that identites come from different places, using different rules. How can it be different from identity hub! And that's how any data retrieval will happen, I dont see how Identity Hub can do it differently.
Virtual directory join engine will translate to identity hub when there is only one repository.
Ashwin