Seen a number of responses about whether or not Vault will pull from
the scratch pool for the catalog backup.  The answer is it depends on
which version.  3.4 and 4.5 Maintenance path: No.  4.5 Feature path:
yes.  

See the Vault Admin Guide, "Ensuring Available Media for Catalog
Backups".

4.5:

You must use a dedicated off-site volume pool for catalog vaulting,
and that volume pool must have unassigned media available (or in the
scratch pool if used).  

4.5, with Feature Pack:

If you use a scratch pool and the catalog volume pool, media is
allocated to the catalog backup volume pool if needed and if media is
available in the scratch pool. 

-- 
Larry Kingery 
          ... and God typed "M-x light" and there was light