1. Open the Exchange Management Console
    • Make sure you are using an Administrator account
  2. Open the Web Management Interface
    • Click Toolbox > Message Tracking
    • Use the same Administrator account details to login
  3. Choose the User to manage
    • In the top-left corner you should see a link that says 'Manage My Organizaiton'; hover over this link and click on 'Select User' from the drop down
    • From the list, search and select the User's name
  4. Edit the user's out of office message
    • A new window/tab should open with basic management options for that user; on the right-hand-side there are a list of shortcuts, click 'Tell people you're on vacation'  This will give you access to both internal and external messages
    • Once complete, remember to save everything and log out

 Spiceworks Article: http://community.spiceworks.com/how_to/show/8375-edit-another-user-s-out-of-office-message-in-exchange-2010-without-editing-permissions