Prepare a grill for medium heat; lightly coat the grates with vegetable oil.
Whisk together the vinegar, mustard, Worcestershire, garlic, 1/2 teaspoon salt and a generous grinding of pepper in a large bowl. While whisking, slowly drizzle in the olive oil until the dressing is smooth and thick. Transfer half to a small bowl and whisk in the parsley and tarragon; set aside. Add the chicken to the dressing remaining in the large bowl and toss to coat.
Grill skin-side down until lightly charred, 2 to 4 minutes. Flip and cook for an additional 5 minutes. Cover the grill and cook until no longer pink near the bone and an instant-read thermometer inserted into the deepest part of each chicken thigh reads 165 degrees F, about 5 minutes more. Transfer to a platter and rest for at least 5 minutes before serving.
While the chicken rests, grill the romaine, turning to slightly char all sides, until the core end is tender, about 5 minutes. Put 1 wedge of romaine on each of 4 serving plates; top with 2 chicken thighs and drizzle with the reserved herbed dressing. ?
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