Place the parsley, cilantro, olive oil, vinegar, oregano, salt, pepper, and red pepper flakes in a blender and process until well blended, scraping down the sides as needed. Place ½ cup in a covered container and refrigerate until serving time. Pour the rest into a large food storage bag, add the chicken thighs, squeeze out the excess air, seal the bag, and refrigerate for 1-12 hours.
Prepare the grill for direct heat and preheat to medium-high. Alternatively, preheat a grill pan over medium-high heat.
Remove the chicken from the bag, discarding any marinade. Brush the grill grates or grill pan with vegetable oil and grill for 4-5 minutes per side or until the internal temperature reaches 160-165°F.