Have you verified the appropriate permission for VSA on Exchange 2003 from the Install and configuring guide are met?
What event ID's are you seeing?
there is also this:
Archiving Task Fails with Event IDs 3305 and 2256
Exact Error Message
The Task 'Mailbox Archiving Task for ...' failed to log on to Exchange server '...' using mailbox 'SMTP:...'. Please ensure the mailbox has not been hidden, that the server is running and that the Vault account has sufficient permissions on the server.
Details:
Symptoms
The Archiving Task Fails with Event IDs 3305 and 2256. The Event ID 3305 lists issues with the System Mailbox.
Running the Deployment Scanner shows Exchange connectivity is good.
Solution
This issue may occur when the Microsoft Exchange Services on the Exchange Server(s) are not Started. Typically, this issue is seen when the Exchange System Attendant Service is Stopped. Verify that all Exchange Services that are set to Automatic are Started on the Exchange Server(s).