In a large pot cover potatoes with salted cold water by 1 inch and simmer, covered, until just tender, about 15 to 30 minutes, depending on size of potatoes. In a colander drain potatoes and cool to warm. Cut the potatoes crosswise into 1/2-inch-thick slices.
Prepare grill. In a small bowl whisk together the oil, cumin, salt and pepper. Brush both sides of the potato slices with the oil mixture. Brush the ears of corn with the remaining oil mixture. Grill the potato slices for 1 minute per side and transfer to a large bowl. If necessary, the grilled potato slices can be halved or quartered into bite-size pieces at this point.
Grill the corn for 3 to 4 minutes rotating once a minute and transfer to a plate. When the corn is cool enough to handle, remove the kernels with a sharp knife. Add the corn to the potato slices. Add the red bell pepper, scallions and 1/4 cup cilantro leaves.
Pour half of the dressing over the vegetables and gently toss to combine. Add the remaining dressing and carefully stir again. Cover and let stand for at least 30 minutes to allow flavors to combine. Sprinkle with remaining chopped cilantro.
Whisk together vinaigrette ingredients.