Gosh, that's like asking for an authentic meatloaf recipe. Even in NC you're going to get almost as many recipes as there are are cooks. In the west they add tomato, in the east they don't.
In it's purest form, traditional NC vinegar based bbq finishing sauce is something like this:
2 cups cider vinegar
3 tbs fresh red chile pepper
1 tsp black pepper, ground medium
2 tsp salt
I usually combine all the ingredients the day before so that the flavors combine. This is a great recipe for very succulent and tender port, combined with a traditional NC cole slaw (cabbage, green peppers, onions, mustard powder, cider vinegar, sugar, salt, pepper, celery seeds).
I have added both tomato paste as well as mustard to the bbq sauce with good results, both of which are "authentic" although adding mustard is more a SC thing.
