In a large bowl, whisk together olive oil, red wine vinegar, smoked paprika, garlic powder, onion powder, oregano, thyme, salt, black pepper, cayenne pepper (if using), and lemon juice until well combined.
Add the chicken thighs to the marinade, ensuring they are fully coated. Cover the bowl with plastic wrap and refrigerate for at least 2 hours, or preferably overnight for maximum flavor.
Preheat your grill to medium-high heat.
Remove the chicken from the marinade and let any excess drip off. Discard the marinade.
Place the chicken on the grill, skin side down, and cook for 6-8 minutes until the skin is crispy and golden brown.
Flip the chicken and reduce the heat to medium. Grill for an additional 15-20 minutes, or until the internal temperature reaches 165°F.
Remove the chicken from the grill and let it rest for 5 minutes before serving. Garnish with chopped parsley.