In a small bowl, whisk together sugar and chili powder. Rub mixture all over pork.
In a blender combine pineapple and soy sauce and blend until smooth.
Add onions and pineapple-soy mixture to a slow cooker on low heat. Place pork in slow cooker, cover, and cook until very tender, 6 to 8 hours (time will vary depending on the slow cooker). The meat should be fork tender.
Shred pork, removing and discarding excess fat.
Pour 1 cup of the juices and the onions from slow cooker into a blender and add ketchup and 3 tablespoons vinegar. Blend until smooth.
Toss shredded pork with pineapple-ketchup sauce to taste.
When ready to serve pork, in a large bowl whisk to combine remaining 2 tablespoons vinegar, mayonnaise, and salt. Add cole slaw and toss to coat.
Serve pork with slaw, buns, and any remaining sauce.