Order the shrimp ahead of time and ask your fish dealer to peel and devein it for you--it's usually free or only costs a fraction more.
Heat 3 tablespoons of the oil in a large, nonstick skillet over medium heat. Add corn and cook, covered, 5 minutes, stirring occasionally. Add shrimp and cook just until shrimp turns pink, about 3 minutes. Remove from heat and transfer to a large bowl. Allow to cool slightly. Whisk together remaining tablespoon of oil with lemon juice and vinegar; and salt and freshly ground pepper to taste. Drizzle over corn and shrimp and toss with tomatoes, scallions, and cilantro leaves. Serve at room temperature or chilled.