I don't know which way your are leaning in your process to go with VTL or Disk, 
but I can share our experience.  We are a smaller environment than you with 
only about 60+ clients that get backed up.  We have Clarion CX500, CX3-20 and a 
EMC VTL.  This was our plan to start with, just put everything to VTL the clone 
to tape for offsite.  Then we implemented this plan and the results where much 
different than what was planned.  The VTL itself is slick, but the upgrades are 
a pain and very pricey.  We ended up moving to de-duplification instead of VTL. 
 Not to promote a vendor specifically, because there are several good ones out 
there, but we bought two Data Domain units for our environment.  We have one at 
HQ and one at DR and they replicate between the two.  Backups are incredibly 
fast, restores are even faster.  We have 110TB of data on our DD530 and are 
looking at adding some more units.  They do exactly what they are supposed to 
do.  We get 20:1 compression overall and our!
  ESX backups are around 55:1.  So if I were you I would seriously look at 
de-dup products before chaining yourself to a VTL.  Just my $.02.

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