The vault option is quite expensive but if you can afford it then it is handy to have.
To answer your question ... NetBackup uses barcodes to set its media IDs so make sure you library has a barcode reader and that you set up the media ID generation rules so that the media id is set correctly.
Once you have used a tape it is known to NetBackup so will be recognised next time it is loaded and you run an inventory
NetBackup uses retention periods so just set these up so that the backups have expired before they are reloaded into the library - they will then get overwritten.
An easy way to manage things is to ensure you have the "Time Assigned" column shoing on your library and Media Section.
When you want to know which tape to remove from the library after a nights backups just go to the robot section and look for ones with a time assigned - they have been used and can be removed
When deciding which tapes can be loaded just go to the Media or Volume pool section and look for ones that do not have a time assigned and are not shown as having a slot number. These have expired and are not the the library and can safely be put back in
Hope this helps