Trim broccoli; wash and break off flowerets. Reserve stalks for other uses.
Arrange broccoli in steaming rack; place over boiling water. Cover and steam 5 minutes. Remove from rack; set aside to cool. Repeat procedure with asparagus and cauliflower.
Place steamed vegetables under cold running water; drain. Combine with zucchini and cucumber in a large plastic bag. Pour marinade over vegetables. Fasten bag securely. Gently turn bag until vegetable pieces are coated. Refrigerate several hours or overnight, turning bag once or twice.
To serve, cut corner from bag, place over colander, and let drain thoroughly. Line salad bowl with Bibb lettuce; empty bag of drained vegetables into bowl. Garnish with tomato wedges.