It depends on your environment on whether to purchase NBU Vault or not. If you write directly to tape AND your shop is of moderate size - flip a coin, you could go either way.
In my environment, NBU writes to a VTL (I need the speed and robotic flexibility it provides), then NBU Vault extracts the images that were created and copies them to physical tape. If I did not have NBU Vault, this task would be most unpleasant and convoluted.