Bring a large pot of water to a boil; add 2 tablespoons salt and the vinegar. Plunge lobster head-first into water; cook 3 minutes (meat will not be fully cooked). Transfer lobster to a cutting board.
Using a kitchen towel to protect your hands, twist off tail and claws; discard body. Twist fan off end of tail, and push meat out of shell. Alternatively, use kitchen shears to cut up length of tail, and pull shell away from meat. Cut tail meat in half lengthwise, and transfer to a plate lined with paper towels; set aside. Separate claws from knuckles; twist and pull off pincers, being careful to keep meat intact. With back of knife, crack knuckle end of claw to loosen shell. Gently remove whole piece of meat; set aside with tail. Using your finger or the handle of a spoon, push knuckle meat out of shell. Wipe any white residue off meat. Meat can be refrigerated in an airtight container up to 2 hours ahead; bring to room temperature before poaching.
Remove peel and pith from tangelo using a sharp knife. Working over a bowl, carve out flesh between membranes, allowing segments to drop into bowl. Squeeze juice from membranes into bowl. Transfer juice in bowl to a separate bowl; reserve juice for dressing.
Put 1/3 cup juice into a small saucepan; bring to a boil. Reduce heat to low, and whisk in butter, 1 piece at a time, until smooth. Add a pinch of salt and the tarragon sprig. Submerge lobster meat in the butter sauce. Poach until opaque and cooked through, 2 1/2 to 3 minutes.
Whisk 3 tablespoons warm poaching liquid into 2 tablespoons reserved juice in bowl; whisk in pepper and 1/2 teaspoon salt. Add lobster to dressing; toss gently to coat. Just before serving, add tangelo segments and tarragon leaves; toss gently to combine. Season with salt.
Arrange 2 lettuce leaves on each serving plate. Place lobster and tangelo segments on top. Spoon dressing over top.