Heat grill to medium. Place the parsley, oregano, and garlic in the bowl of a food processor and pulse until roughly chopped. Add the vinegar, oil, cayenne, and 1/2 teaspoon of the salt and pulse to combine; set aside.
Season the steak with the remaining salt and the pepper. Grill to desired doneness, about 4 minutes per side for medium-rare. Transfer to a cutting board and let rest 5 minutes before slicing.
Meanwhile, as the steak cooks, toss the potatoes in a large bowl with 1/3 cup of the herb sauce. Place them on the grill in a single layer. Cook, turning once, until tender, about 4 minutes per side. Divide the steak and potatoes among individual plates and serve with the remaining herb sauce on the side.
Tip: For a quick boost, toss this fresh, flavorful sauce with pasta or spoon it over chicken, fish, or pork.