Denda,
Below is a response I provide nearly 2 years ago in this forum. Parts of it may not apply to you, but I hope you find this useful:
==============================
The Vault process has everything to do with how images are stored on the VTL - especially if multiplexing to the VTL. For example on the VTL, tape #001 contains image_001, image_002, image_003. Tape #002 contains image_001, image_003, image_004.
When you start the vault process, it goes to tape #001 and gets the image_001. BUT, it knows that image_001 is also on tape #002. It will dismount tape #001, mount tape #002 and continue vaulting image_001. Once that is completed, it dismounts tape #002, mounts tape #001 and proceeds to image_002. Since it is contained strictly on tape #001 - no problem. Then it vaults image_003. Again, it performs the same process as it did for image_001. This is what will kill you when vaulting. There is a fine line in trying to optimize writing to the VTL AND using Vault.
Because of the possible mounts / dismounts (even virtually), fragment size is very important. If you did set fragment size, then going back to my example, when mounting tape #001 to begin vaulting image_002, NetBackup is going to have to scan tape #001 from the beginning to get to image_002 starting point.