1. In small bowl, whisk together vinegar, sugar, red pepper flakes, salt and pepper. Stir until sugar dissolves.
2. Score pork with crosshatch pattern, making diamond shapes, cutting through skin but no more than 1/4 inch deep.
3. Pat dry, then rub with salt & pepper. Let stand at room temperature for 1 hour.
4. Scatter onions and garlic over bottom of slow cooker. Put pork on top and add water. Then pour prepared vinegar sauce on top.
5. Cook on low for ~8 hours or until tender. Remove meat and let cool slightly.
6. Transfer to cutting board. Use 2 forks to shred meat while removing fat and gristle.
7. Remove garlic and onion and mash into paste.
8. Return pork and paste to slow cooker and stir in honey, mustard, and Worcestershire sauce.
9. Season with salt & pepper as needed and cook an additional 1 hour on low in slow cooker.