Dissolve gelatin in boiling water; stir in cold water, vinegar, and salt. Chill until consistency of unbeaten egg white. Fold in corn, green pepper, pimiento, and onion. Spoon into a lightly oiled 4-cup mold. Chill mixture until set. Unmold salad onto a lettuce-lined serving plate, and garnish with a large dollop of mayonnaise.