Whisk vinegar, water, sugar, and salt together in medium saucepan and bring to boil over high heat. Cook, whisking frequently, until sugar and salt dissolve, 2 to 3 minutes.
Meanwhile, transfer onions to large jar or bowl. Pour hot pickling liquid over onions and let cool for 20 minutes. Cover and refrigerate until onions turn pink and are seasoned throughout, at least 8 hours. Strain onions and serve. (Onions can be refrigerated in pickling liquid for up to 2 weeks.)