Don't use it.
I mean it. Do not use auto-reply vacation email setting because there is no effective way to use it.
Getting "OOO I'm on vacation" email from a guy who has a desk next to me is just ridiculous. I can visually see his empty desk now and he told me he will be on vacation before he left. Vacation auto-reply email function is not smart enough to determine if recipient knows sender's vacation schedule or not because there is no such a technology to look into my brain from any mail exchange servers delivering the shit. What is even worse is that I'm getting the same auto-reply message every single day! I'm getting old and have a short memory but I not that old to forget you're out or not each day. I DID send an email to you to let you read it later and I knew you're off. (oh wait, are you using vacation mail to prevent getting more email in your inbox? smart ass).
Email (technically its delivery protocol SMTP and client access protocol IMAP and POP) is designed in a way to allow us asynchronous communication unlike the usual telephone invented by Alexander Graham Bell. Your recipient might see your email or might not. There is no guarantee to have your recipients read your email. Conversely it is totally fine that a sender doesn't get any reply from recipients during his or her two-week vacation. Getting no reply clearly tells me you did not read (or you did not pay attention) my email. I'll find another way around if I need to get your reply like making a telephone call or trying to reach via IM, whatever. It is a absolutely sender's responsibility to complete a task communicating with recipients.
I'm telling this now because I think you never noticed how annoying the auto-reply is.
Photo Credit: Beach Umbrella by Timothy Valentine http://www.flickr.com/photos/el_ramon/4686431861/