Heat grill to medium-high.
In a small bowl, combine the vinegar, ginger, thyme, 3 tablespoons of the oil, 1/4 teaspoon salt, and 1/4 teaspoon pepper; set aside.
Season the salmon with 1/2 teaspoon salt and 1/4 teaspoon pepper. In a large bowl, gently toss the onions, peaches, the remaining 1 tablespoon of oil, 1/2 teaspoon salt, and 1/4 teaspoon pepper.
Grill the salmon and the onions, until the salmon is opaque throughout and the 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.