You can't buy empty Vertuo capsules. Nespresso's Vertuo capsule design is protected, so no one sells an empty compatible base. Instead, you reuse the capsules you've already brewed.
The barcode printed on the base of each Vertuo capsule tells your machine how much water to use, what temperature to brew at, how long to infuse and how fast to spin the capsule. That's why the base has to be a genuine Vertuo one — and why every reusable Vertuo product on the market is a lid rather than a whole capsule.
Save a handful of empties and you can rotate them indefinitely.