In a small saucepan over high heat, heat the white vinegar, water, sugar, and salt. Whisk until the sugar and salt are dissolved and the mixture starts to simmer, about 2 to 3 minutes.
Place the onions in a jar or bowl. Pour over the brine and allow it to sit for at least 30 minutes. Transfer to the refrigerator and store refrigerated for up to 1 month.