Heat grill to medium-high. In a small bowl, combine the vinegar, ginger, thyme, 3 tablespoons of the oil, and ¼ teaspoon each salt and pepper. Set aside.
In a large bowl, gently toss the onions, peaches, remaining 1 tablespoon oil, ½ teaspoon salt, and ¼ teaspoon pepper.
Season the salmon with ½ teaspoon salt and ¼ teaspoon pepper.
Grill the salmon and onions until salmon is opaque throughout and onions are tender, 5 to 6 minutes per side.
After flipping the salmon, place the peaches on the grill and cook until tender, 3 to 4 minutes per side.
Drizzle the salmon with the vinaigrette and serve with the onions and peaches.