Put the vinegar and garlic in a small bowl. Put the shallot in another small bowl and fill with ice water. Let sit for 10 minutes.
Meanwhile, bring a small pot of water to a boil over high heat. Add the corn kernels and cook for about 30 seconds. Drain and set aside.
Heat 2 Tbs. of the oil in a 10-inch sauté pan over high heat until hot. Add the shrimp and season with salt. Cook, tossing frequently, until the shrimp turn pink and light golden brown, 2 to 3 minutes. Transfer to a bowl and set aside to cool briefly.
Whisk the remaining 1/2 cup oil into the vinegar mixture. Drain the shallot. Put the shallot, corn, tomatoes, chile, and marjoram in a large bowl, season with salt, and toss with about 3 Tbs. of the vinaigrette.
Put the arugula in a separate bowl, season with salt, and toss with just enough vinaigrette to lightly coat the greens, about 3 Tbs.
Taste the corn mixture and the arugula and add salt if necessary. Toss any remaining vinaigrette with the shrimp. Transfer the arugula to a large platter or four plates and distribute the corn mixture on top of and around the arugula, making sure that the mixture doesn’t weigh down the greens. Tuck the shrimp in and around the salad and serve immediately.
To make this dish more filling, serve an avocado toast on the side: Rub warm toast with a clove of garlic, spread coarsely mashed avocado on top, season with salt, and drizzle with olive oil.