Preheat a grill or broiler to high.
In a blender, combine vinegar, jalapeno, garlic, and bay leaves puree until smooth. Transfer to a bowl and add the herbs. Whisk in the olive oil and season with salt and pepper. Mix well and set aside.
Season the flank steak with olive oil, salt, and pepper and grill/broil the steak for 2 to 3 minutes per side for medium rare. Slice the steak thinly, across the grain on the bias and serve with the chimichurri sauce.