Preheat the oven to 350°F. Roast the corn for about 30 minutes, until tender. Remove from the oven and let cool slightly. When cool enough to handle, shuck and corn and cut the kernels off the cobs. Toss together the corn, watermelon, cucumber, feta, red onion, and basil in a large bowl.
Whisk together the vinegar and olive oil in a small bowl and season with salt and pepper, to taste. Drizzle the dressing over the salad and toss gently to coat.