Place white vinegar, water, sugar, and salt in a small saucepan and bring to a simmer. Stir until the sugar and salt completely dissolve into the liquid.
Meanwhile place onion slices in a large jar or bowl. Pour the hot liquid over the slices, and make sure the onion is completely covered in liquid. Let the onion rest this way for at least 30 minutes, however the flavor will continue to improve the longer it is in the pickling liquid.