Turn Instant Pot sauté setting to high. Add 2 Tbs. oil and sear 4 to 5 ribs on all sides, Place browned short ribs on a plate. Repeat with remaining ribs.
Pressure-cook on high for 45 minutes, followed by naturally releasing steam for 15 minutes. Release remaining pressure by adjusting pressure valve to venting position. Open lid.
Remove short ribs to a bowl. Pour sauce into a saucepan and skim off fat. Whisk together cornstarch and 1/4 cup water. Whisk mixture into sauce and simmer until thickened slightly. Stir in rice vinegar.