vault duplication jobs

"...as the drives were often maxed out..."

Do you meant all 18 were in use at once?  Adding more tape drives to a single server will not speed up the backups, you would need to add more media servers and distribute tape drives among them.  I suspect you have very low throughput speeds if you really only have one master/media server.

The backup data comes from the client to the media server and then out to the tape drive.  Assuming fiber channel HBA's, you will likely have either 4Gbit or 8Gbit data path in your HBA's.  The combinations of NIC speeds, SAN speeds, PCI bus speeds, etc. plus overhead make a single media server too slow to stream data to more than about two physical tape drives fast enough to keep your tape drives moving without having to stop, wait for more data, reposition the tape to the correct block to write, write data in the buffer, stop, etc.  This "shoe-shine" effect just wears your tape drives down.

VTLs are different because they don't need to worry about tape repositioning during backups.  But you are still throttling your data through a small pipe that must also serve more than two physical tape drives, plus all the data coming into the VTL during backups.

In my opinion, without even seeing your policies or other configs, this is why your duplications are taking too long.  You need to increase the amount of data you can push from the VTL to the tape drives simultaneously.  That can only be done by adding media servers and moving some of the throughput load off of the current server.