Place salmon in large zip-top plastic bag and pour ½ cup vinaigrette over salmon. Seal bag and refrigerate 30 minutes.
Prepare outdoor grill for direct grilling over medium-high heat; oil grill rack. Place salmon on hot grill rack, skin side up, and cook 3 to 5 minutes per side, or until internal temperature reaches 145°. Separate skin from salmon while removing salm
To serve, distribute spinach, onion, tomatoes, cucumber, cheese, and salmon fillets over 4 serving plates. Serve with remaining vinaigrette. Garnish with chives, if desired.