Heat a medium saucepan over high heat. When it’s hot, add the bacon and cook, stirring occasionally, until browned and crisp, about 5 minutes. Use a slotted spoon to transfer the bacon to paper towels and set aside.
Discard all but 1 tablespoon of the bacon drippings and return the pan to medium heat. Add the onion and garlic and cook until softened and golden brown, about 3 to 4 minutes.
Stir in the corn, lima beans, and black-eyed peas and season well with salt and pepper. Add the water and vinegar and stir to combine. Reduce the heat to medium low and cook until the beans are tender, about 5 minutes. Remove from heat and stir in the reserved bacon and butter. Taste and adjust the seasoning as necessary.
For the fish:
Heat a grill or a seasoned grill pan to medium high (about 375°F). Pat the fish dry with paper towels, brush both sides with vegetable oil, and season well with salt and pepper.
Place the fish skin-side down on the grill or in the pan and cook until the flesh is firm and just opaque in the center, about 4 to 5 minutes per side. Remove from the grill, immediately top with the compound butter, and serve over the succotash.