Whisk the vinegar, sugar and 1/2 teaspoon salt in a large bowl, then whisk in the olive oil. Drain the onion and pat dry, then add to the dressing. Stir in the white beans.
Bring a saucepan of salted water to a boil. Add the green and/or wax beans and cook until crisp-tender, 4 to 5 minutes. Drain the beans, then plunge into a bowl of ice water to stop the cooking. Drain again, pat dry and add to the salad. Marinate at room temperature, tossing occasionally, about 1 hour.
Before serving, stir the parsley and chives into the salad and season with salt and pepper.