Wouldn't the latter option be kind of pointless? If the Car Concierge was added like that, then it would be like the Wishlist system we already have in game but with extra steps (and possibly a fee too).
It would for the most part, but Polyphony Digital isn't a stranger to adding pointless things. Though I don't recall if the wishlist system allows you to wishlist a car anytime you want or if it has to be in the dealership to do so.
So I think it's safe to assume that you pay for a Vintage car to be ordered, and it arrives X days later. A user in this thread suggested that this feature could also mean that you can choosethe colours of the cars you order (factory colours), and I really hope that's the case...
I really hope that it's absolutely that, I'm just thinking of a worse case scenario as Polyphony has a tendency to add things with a "catch" in the current GT. Which hurts my optimism whenever a new feature is announced.