One area I've used VV's is for remote vaulting.  Yes, you can write
directly to a tape drive at the remote site, but over a slower connection,
trying to send data to a tape will cause LOTS of backhitching on the drive,
reducing tape performance (which probably isn't a problem), and reducing
the useful life of the tape cartridge (bad).  VV's allow you to "land" the
data on disk, the allow the local TSM Server to migrate it to tape.  You
also aren't occupying a tape drive at the remote site for the (extended)
duration of the job.

And a TSM DB Backup takes (at least) one volume, so with physical
cartridges, that's a whole tape.  With VV's, you're only using the actual
capacity of the backup, which is more efficient on space.  If you're doing
multiple backups per day, or lots of incrementals (for example, having
logmode set to rollforward on a busy server), being able to stack multiple
VV's on a physical tape will greatly reduce the tape cartridge resources
you need (I'm seeing installations getting over 2TB on the new 3592/TS1120
drives - for a 60GB TSM DB Backup, that's VERY wasteful).

With all of the features in TSM, there are a number of them that don't work
for specific situations.  Simultaneous writes on backup/migration, virtual
volumes, NDMP backups, 3rd mirrors of DB and Log volumes, adding
documentation to your Prepare file - lots of features that don't always
make sense to use.  But they're there, if you want/need them.

Nick Cassimatis