1. Pulse strawberries and sugar in a food processor until finely chopped. Transfer berries and juice to a large bowl; stir in vinegar. Cover and let stand 2 1/2 hours.
2. Strain berries and juice through a double thickness of rinsed and squeezed dry cheesecloth; discard solids. Transfer liquid to a bottle with a tight-fitting lid. Store in refrigerator up to 3 weeks.