Preheat a grill to medium high. Bring the vinegar, sugar, mustard and lemon zest to a simmer in a medium saucepan. Cook, whisking occasionally, until syrupy, about 3 minutes. Add the nectarines and cook until just softened, 3 to 5 minutes; keep warm over low heat.
Meanwhile, toss the fennel and red onion with the olive oil in a large bowl; season with 1/2 teaspoon salt and a few grinds of pepper. Brush the grill grates with olive oil. Grill the vegetables (with the fennel perpendicular to the grates to keep them from falling into the grill, or use a grill basket), flipping, until marked and crisp-tender, 5 to 7 minutes. Transfer to a large bowl. Add the lemon juice and parsley; toss to combine.
Season the pork chops with salt and pepper. Brush the grill grates with more olive oil. Grill the pork until marked and just cooked through, about 4 minutes per side. Serve with the nectarines (discard the lemon zest) and the fennel-onion mixture.