Grill the corn, or place over a gas burner set to medium heat, until charred, 2 to 3 minutes per side. Cool for 10 minutes, then cut the kernels away from the cob and reserve for later use.
Heat 3 tablespoons of the oil in a large saute pan set over medium heat. Saute the onions until slightly softened, 2 to 3 minutes. Add the garlic and saute for another 2 minutes. Reduce the heat to low, add the tomatoes, snap peas and black-eyed peas, and then season with salt and pepper. Cook just to meld the flavors together, 2 to 3 minutes.
In a large bowl combine the cooked vegetables, corn and vinegar, and mix. Season with salt and pepper, and then stir in the remaining 2 tablespoons oil to serve.