For now, I've set my unvaulted Policies all to a few generic Volume Pools. Those Policies I do Vault I've assigned to individual Pools, or to Pools shared among a few of them.
I was getting much better results on my backup times when I had some of the Vaulted Policies writing to generic Pools, but some admins on my team reported vault failures and partial failures.
I didn't investigate the individual errors thoroughly (there's too much else to do), but am guessing/assuming they might have been related to tape availability. My backups are definitely writing a lot slower with these major policies separated, but still finish within my nightime time window deadlines.
It's diffucult with LTO2 drives and 100mb/s connections from clients, because without writing multiple policies to a tape simulataneously I can't combine enough streams to hit the drives' native speed (30mb/s).
My concept of engineering a Netbcakup environment, and probably any kind of backup environment, is it's a juggling act balancing optimal backup, vault, and restore speeds. At least in my data center restore speeds are still a very low priority. I mainly just have to worry about backup and vault legal requirements to get everything on tape and out the door.