In a small pot over medium heat, combine vinegar and sugar. Bring to a boil and simmer until syrup is reduced by about half, 8-12 minutes. Remove from heat and cool slightly; stir in sriracha sauce and set aside. After serving, refrigerate any leftovers for future use.
Preheat oven to 450°. Set pan over medium-high heat and add oil. When warm, add chopped kimchi and cook, stirring, until warm - about 30 seconds. Add eggs and cook for 3-4 minutes over low heat, gently lifting the egg/kimchi mixture away from bottom of pan. Do not allow to brown on bottom! Transfer pan to middle rack of oven and cook 3-4 minutes, checking after 3 minutes. Cook just until set; serve immediately, garnished with toasted black sesame seeds. Drizzle sriracha syrup over frittata at table. 3 large servings.