How do I setup an auto-reply email vacation message?

Vacation messages can be setup on google mail.

Click on the "settings" button on the right of your google email screen (it looks like a gear wheel). When the menu pops up, choose "settings".

A new page will appear with various settings. Close to the bottom of this page, click on the selection for "Vacation responder on". Set your vacation start and end date, as well as the message, then click on "Save Changes" at the very bottom of the page.