Need an explanation of what the local EV db's are?

These DB files are used for Offline Vault (if you are using Enterprise Vault 2007 clients or below) or Vault Cache (if using Enterprise Vault 8+ Server and clients)..... The db files are basically PST files that contain local copies of the messages contained in the users archive.

If you are using Enterprise Vault 8 and Vault Cache then the DB files are created on the server first and then downloaded to the client which is then placed in their user profile on the machine, once downloaded the DB files are then deleted from the server

If you are using Enterprise Vault 2007 and Offline Vault then the DB files are only created on the users machine and download.asp downloads each individual message to the DB files.

If you just go ahead and delete the files straight off then what will happen is that the next time outlook opens it will simply start rebuilding them again, and users will be affected if they expect to be able to open items offline and now start getting errors.

This is all controlled by the policy, in Enterprise Vault 8 it will be in the Desktop Policy for the users, in Enterprise Vault 2007 it will be in the mailbox archiving policy under the Advanced tab in the Desktop drop down menu