With your product listed you may now configure your first vendor!
After rezzing your vendor and granting debit permissions, click on it and choose "Setup" on the dialog that appears and then open the link given to you in local chat.
Now in the "Product" field, start typing the product name, a dropdown with the matching options will appear. Select the desired product.
That's all you need to associate the product with the vendor. You may click the "Save" button and the vendor will be updated with the corresponding price!
Check the vendor inworld and do a test purchase by paying it, the product should be delivered to you.