Slice the kernels off 2 ears of corn. Grate the remaining ear on the large holes of a box grater.
Bring 1/4 cup water, the vinegar, sugar and a big pinch of salt to a boil in a small saucepan over high heat. Remove from the heat; add the sliced red onion and set aside to pickle, at least 10 minutes.
Heat 1/2 tbsp vegetable oil in a medium saucepan over medium-high heat. Add the diced red onion and cook, stirring occasionally, until softened, about 4 minutes. Add the corn kernels and cook, stirring, 2 minutes. Reduce the heat to medium and add the evaporated milk and grated corn. Simmer until thick, 5 to 7 minutes; season with salt and pepper. Cover and keep warm over low heat.
Heat the remaining vegetable oil in a large nonstick skillet over high heat. Season the scallops with salt and pepper. Add to the skillet and cook, undisturbed, until browned on the bottom, about 3 minutes. Flip and cook until cooked through, about 1 more minute.
Divide the corn among bowls and top with the scallops. Drain the pickled onion, reserving a teaspoon of the liquid. Toss the onion and liquid with the tomatoes, basil and parsley; season with salt. Add to each bowl.