Yes, Vault Drive does contain Vault/FLARE/System LUNs but they are not a part of user LUNs which resides on user created RAID Groups. Vault LUN helps CLARiiON to run and any impact caused by user LUNs created on the Vault Drive directly and adversely impact the performance of the entire array.  In a Gist: Yes, you can replace the 146GB disk with 300GB Disk but try not to create any high IO Intensive LUN on it, if you want to avoid Poor Performance Issue.  You may want to check more on Vault Disk Contents: CLARiiON VAULT Disk Contents and Vaulting Operations
I have an AX-4 with a faulted disk.  It's Enlcosure 0, Disk 2 (10k, 146gb), which is in the Disk Processor Enclosure.  I have a replacement drive (10k, 300gb) for the SAN, but can I use it to replace the DPE disk?  There are 4 disks on the DPE that have a 'do not remove' graphic, thus my worry.  Here is my event log:
Replace faulted disk in DPE?