The attached VBScript allows you to set an Out-of-Office reply for another user on an Exchange 2003. You'll need the required permissions on the Exchange server to set a user's out-of-office (either Mail Recipients or User Options Management roles).
I found the script in the comments section of this Technet blog article. Special thanks go to the author (Bill Prentice) for sharing.
This script is only for Exchange 2003. Starting from 2007, the Powershell cmdlet Set-MailboxAutoReplyConfiguration is available.