When deploying Enterprise Vault (EV) to archive from an Exchange server, a pre-requisite is to grant the Vault Service Account the proper rights to Exchange. There are 2 ways in which this can be achieved. Either using the Exchange System Manager or using the Exchange Management Shell (Powershell).
The Powershell method can only be used if the Exchange Organization is at least Exchange 2007 (i.e. a mixed environment where Exchange 2003 servers co-exist with Exchange 2007) and the scripts are run from an Exchange 2007/2010 console and remotely executed against the Exchange 2003 server.
If your Exchange environment contains Exchange 2003 servers within an Exchange 2007/2010 organization you can use the Powershell scripts (Script can be run remotely from a server using PowerShell v2.0) follow the steps within Technote " Using Powershell scripts to assign Exchange Server permissions for the Vault Service Account" referenced below.
If your Exchange environment contains only Exchange 2003 and/or Exchange 2000 you cannot use the Powershell scripts. You will have to use the steps within this Technote " How to grant the Symantec Enterprise Vault (EV) Vault Service Account (VSA) the correct permissions On Microsoft Exchange Server 2003 and Microsoft Exchange 2000 Server" referenced below.