We use Parking Vert every year and they are un-numbered space as JPC says.
We always order ours as they get delivered along with the rest of our order allocation, but you can buy them from ACO ticket sellers on the Chemin Des Boeufs on the Saturday. Never seen them there earlier than that though.
Whenever the Vert site is reasonably full of vehicles another field (usually on the other side of the road to the Stella Bar, just under the bridge) is opened to take the overspill so shouldn't be a problem with over-subscription.