Pulse together parsley, cilantro, oil, vinegar, and garlic in a food processor until very finely chopped. Transfer mixture to a small bowl, and stir in softened butter until combined. Measure 3 Tbs. herb-butter mixture into a small bowl; stir remaining herb-butter mixture into mashed potatoes in saucepan. Transfer mashed potatoes to a serving bowl. Swirl reserved herb-butter mixture into top of mashed potatoes. If desired, sprinkle with crushed red pepper.