You can only use that feature if you have an Ebay store...your auctions end when they are supposed to, there's just a flag with a little standardized announcement about 'seller will be on vacation' etc. etc.
If you don't have a store you could always revise with a line of text (as long as there are no bids) but if someone has the item on their watch list they probably won't go back to read the description again, therefore still not knowing you're on vacation....my friend just ended 10 of her new auctions because she just realized she'd still be gone long after they were over.
