Cook leeks, butter, and 1 teaspoon kosher salt in a small saucepan over low, covered, uncovering often to stir, until leeks are very tender, 15 to 20 minutes. Remove from heat.
While leeks cook, stir together tomatoes, vinegar, and remaining 1 teaspoon kosher salt in a small bowl; let marinate at room temperature at least 10 minutes or until ready to use.
Heat oil in a large skillet over high. Add corn; cook, stirring often, until corn is warmed through, 3 to 4 minutes. Add shrimp; cook, stirring often, until shrimp turn pink and are cooked through, 3 to 5 minutes. Remove from heat.
Add leek mixture and tomato mixture (with juices in bowl) to shrimp mixture in skillet; stir to combine. Season with pepper and additional kosher salt to taste. Sprinkle with basil. Serve with warm crusty bread smeared with a thick layer of butter and a sprinkle of flaky sea salt.
