Place the pork in the slow cooker. In a bowl, mix soy sauce, fish sauce, brown sugar, garlic, ginger, red pepper flakes, and black pepper. Pour over pork.
Cook on low for 8 hours or until the meat is fork-tender. Shred the pork using two forks.
In a bowl, combine carrots and cucumber. In a separate small bowl, stir together rice vinegar, water, sugar, and salt. Pour over vegetables and let sit for at least 30 minutes.
To assemble, spread mayonnaise on each toasted baguette or roll, add shredded pork, pickled vegetables, and top with fresh cilantro.
Serve immediately and enjoy!