Place shredded cabbage, carrots, and any optional vegetables in a large bowl.
In a separate bowl or jar, whisk or shake together vinegar, sugar, oil, celery seed, mustard seed, salt, pepper, and red pepper flakes until sugar dissolves.
Pour dressing over the vegetables.
Toss thoroughly until all cabbage is evenly coated.
Cover and refrigerate for at least 2 hours (preferably overnight).
Stir well before serving and drain slight excess liquid if needed.