Place potatoes in a large pan of boiling water; cook 8 minutes or until tender. Add onions and sliced fennel; cook 3 minutes or until onions are tender. Drain and plunge into ice water; drain.
Toss vegetables with oil. Place vegetables on grill rack coated with cooking spray; grill 4 minutes on each side or until tender and lightly browned. Sprinkle with 1/4 teaspoon salt, 1/4 teaspoon pepper, fennel fronds, and parsley.
Sprinkle salmon with the remaining 1/4 teaspoon salt and remaining 1/4 teaspoon pepper. Place salmon on grill rack coated with cooking spray, and grill 5 minutes on each side or until fish flakes easily when tested with a fork or until desired degree of doneness. Place 1 salmon fillet onto each of 4 plates. Toss pea tendrils with Preserved Lemon Vinaigrette. Place about 1/2 cup pea tendril mixture over salmon. Add 1 1/2 cups grilled vegetables to each serving, and serve immediately.