Easy Crockpot Pulled Pork

1 large Vidalia onion, sliced thin

2 tablespoons brown sugar

1 tablespoon paprika

2 teaspoon kosher salt

½ teaspoon freshly ground black pepper

1 (4-6 lb) boneless pork butt or shoulder

¾ cup cider vinegar

4 teaspoons Worcestershire sauce

1 ½ teaspoons crushed red pepper flakes

1 ½ teaspoons sugar

½ teaspoons dry mustard

½ teaspoon garlic salt

¼ teaspoon cayenne pepper