If you do not have a readable barcode on your cover artwork, the Bookvault US facility will add one. This will be added to the rear of your cover artwork in the same location as KDP would add a barcode;
The standard dimensions of the barcode produced are; 38.1mm wide x 25.4mm tall
This is positioned 6.35mm (0.25in) from the bottom of the cover, and 6.35mm (0.25in) from the spine.