In a blender, combine the vinegar, oil, garlic and thyme and puree until smooth. In a large glass or ceramic dish, pour the marinade over the steak. Let stand for 5 minutes.
Heat a grill pan. Season the steak with salt and pepper. Grill over moderately high heat, turning once, until medium, about 8 minutes per side. Transfer the steak to a board and let stand for 10 minutes. Slice the steak, transfer to plates and serve.