Make vinaigrette: Whisk vinegar, lemon juice, mustard, salt and pepper in a small bowl until salt has dissolved. Slowly add vegetable and olive oils, whisking constantly until blended and thickened. Taste and season with more salt and pepper, if desired. Makes about 1/2 cup. (Vinaigrette may be made up to 2 days ahead. Keep covered and chilled. Bring to room temperature and whisk well before using.)
Make salad: Place pine nuts in a small nonstick skillet over medium-low heat. Cook, shaking skillet often, until nuts are golden brown and fragrant, 2 to 3 minutes. Transfer to a bowl to cool.
Cut a slice off tops and bottoms of oranges. Use a sharp knife to cut away peel and white pith, starting at the top and following the form of each orange. Working over a large bowl to catch juices, slice out whole orange segments from surrounding membranes. Set segments aside and squeeze as much juice as possible from membranes into bowl.
Remove and discard stalks from fennel. Quarter each bulb, remove tough core and pull apart layers. Thinly slice each layer and transfer to bowl with reserved orange juice. Toss well to prevent discoloration. Drain fennel before adding to salad.
To assemble salad, place half of vinaigrette in a large bowl. Add watercress and fennel. Top with a tablespoonful of dressing and toss mixture to coat. (Pour in more dressing if needed. The watercress should be lightly coated but not wet.) Divide salad among 8 serving plates. Garnish with orange and toasted pine nuts. Serve immediately.