Preheat a grill or grill pan over medium-high heat.
Put the zucchini, bell peppers and asparagus in a large bowl and toss with a scant 1 tablespoon of the olive oil and scant 1/4 cup of the vinaigrette, making sure everything is nice and coated. Transfer the coated veggies to the grill. Add the onions directly to the grill and give them a good brush with the vinaigrette and olive oil mixture. Grill, turning the veggies every 2 minutes, until tender and well-marked, about 8 minutes. Remove to a serving platter.
Pat the salmon dry with a paper towel. Reserve 2 tablespoons of the vinaigrette for serving, then brush the salmon on each side with 1/2 tablespoon of the remaining vinaigrette; drizzle on both sides with the remaining olive oil. Grill the salmon for 2 minutes, then rotate 45 degrees and cook for another 2 minutes. After 4 minutes total, flip the salmon and grill on the other side for 2 minutes, then rotate and cook for 2 minutes more. Brush with some of the remaining vinaigrette while cooking if needed.
Transfer the salmon to the serving platter with the vegetables. Drizzle everything with the reserved vinaigrette. Sprinkle over the feta, olives, and parsley. Serve immediately.