Cut chicken thighs into 1-inch pieces and let sit at room temperature for 10 minutes. Toss chicken in a bowl with salt, vegetable oil, and 1/3 cup cornstarch until evenly coated. Air fry at 375°F (190°C) for 10 minutes, flipping halfway. Transfer to a wire rack to rest. In a saucepan, combine ketchup, rice vinegar, soy sauce, brown sugar, and 4 tablespoons water. Bring to a boil. Stir in cornstarch-water mixture and cook until thickened, about 2 minutes. Let cool slightly. Spray air fryer with oil and add mixed vegetables. Air fry at 375°F (190°C) for 5-7 minutes until tender-crisp. Combine cooked chicken and vegetables in a large bowl. Pour sauce overtop and toss to coat. Serve immediately.