For dressing, in the work bowl of your food processor, process vinegar, and tarragon. Slowly pour in the oil. Stop the machine and taste. Adjust the seasonings. Remove the lobster meat from the shell, slice the tail meat but leave the claws whole if possible, slice the potatoes. Toss the greens with 1/4 cup of dressing and put in the center of a plate. Toss the tomatoes, potatoes, and green beans with the oil and season with salt and pepper. Arrange in an attractive manner on the greens. Carefully toss the lobster with 1/4 cup of the dressing, and place on top of the salad. Smear the plate with the remaining dressing.