Vidalia only grow in that one spot because Vidalia onions are regular sweet onions grown in a sulfur poor environment. The sandy soil of Vidalia, Georgia causes sulfur to run down through the soil into the water table instead of settling near the surface to be used by plant life. This results in low VOSC content in the onions reducing the burn from them. The most notable of these VOSCs is Allicin ,a compound which when exposed to oxygen, acts basically like tear gas to us humans.

Edit: de-shatnerized