Mix the vinegar, brown sugar, 2 tablespoons water and 1 1/4 teaspoons salt in a large bowl. Add the black-eyed peas and red onion, toss well and set aside to marinate.
Meanwhile, preheat a grill to medium. Brush the cabbage wedges all over with 2 tablespoons olive oil and season with salt and pepper. Grill the kielbasa and cabbage, turning halfway through, until charred and the cabbage is crisp-tender, 8 to 10 minutes.
Chop the cabbage into bite-size pieces and add to the black-eyed pea mixture. Stir in the parsley and remaining 1 tablespoon olive oil; season with salt and pepper and toss. Divide among plates. Slice the kielbasa and add to the plates. Serve with mustard.