In a medium saucepan, combine the vinegar, water, sugar, and salt. Heat over medium heat stirring frequently until the sugar and salt are dissolved. This will take about 5 minutes.
Once the sugar and salt have fully dissolved, turn off the heat and let cool slightly for 5 minutes.
While the pickling liquid is cooling, half the red onion and thinly slice into half-moon shapes. Add them to an air-tight, glass container such as a mason jar or refrigerator storage container.
Pour the slightly cooled pickling liquid over the top of the onions until just covered; they do not need to be fully submerged.
Cover, store in the refrigerator for one hour before serving.