Place the sliced vegetables in a colander lined with a paper towel. Generously sprinkle with salt and let sit for at least 30 minutes to allow any liquid to drain off. Rinse well and set aside to drain again.

In a saucepan, heat sugar, vinegar, water, soy sauce and pepper flakes, stirring until sugar has dissolved. Set aside to cool slightly.

Add vegetables to 2 quart-sized or 4 pint-sized canning jars or plastic containers. Pour vinegar mixture over vegetables, cover with a lid and refrigerate overnight before serving.

Store in refrigerator for up to two weeks.