Spray 4 qt slow cooker with cooking spray. In a 10" skillet, heat oil over medium heat. Cook pork in oil 6 - 8 minutes, or until brown on both sides. Place pork in slow cooker. Cover; cook on low heat setting 7 - 8 hours or until pork pulls apart easily with fork.
Remove pork from slow cooker; shred pork. Reserve 1/4 cup liquid in slow cooker; discard remaining liquid. Stir pork, salsa, chili powder, and cumin into liquid in slow cooker.
In medium bowl, stir together mayonnaise, vinegar, and salt. Gently stir in avocado and onion. To serve, place pork in tortillas; top with avocado-onion slaw and chopped cilantro.