"To soothe a sunburn spray white distilled vinegar on skin, repeating as often as you like. Let it dry naturally, do not rub off. Ice-cold white distilled vinegar will feel even better, and may prevent blistering and peeling." Courtesy of Vinegar Tips
Thanks to a friend of mine who teaches science I learned that vinegar is made of water and acetic acid, so it will moisturize and soothe your sunburn. Acetic acid is called an NSAID (Non-Steroidal Anti Inflammatory Drug), which helps reduce the swelling of the damaged tissue and relieves the pain.
You can also pour a complete small size bottle of white vinegar in a bath of luke warm water and soak in it for about 15 minutes or longer if desired, it will relieve the pain of sunburn immediately and the redness that comes the sunburn and helps prevent peeling. You can also apply vinegar before going out in the sun and you will lessen the chances of getting sunburn! Courtesy of The Vinegar book.
Here are a few other helpful tips for sunburns
Clean and Simple Sunburn Remedies
eHow: How to Treat a Sunburn with Vinegar