Heat grill to medium.
Pulse the parsley, oregano, and garlic in a food processor until roughly chopped. Add the vinegar, oil, cayenne, and ½ teaspoon salt; 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; let rest 5 minutes before slicing.
Meanwhile, toss the potatoes in a large bowl with ⅓ cup of the herb sauce.
Place the potatoes on the grill in a single layer. Cook, turning once, until tender, about 4 minutes per side.
Divide the steak and potatoes among plates and serve with the remaining herb sauce on the side.