Make the pickled vegetables: In a medium saucepan, stir together the vinegar, sugar, salt, and water over medium high heat. Simmer and stir until the sugar and salt have completely dissolved, then pour over the carrots and daikon radish in a large bowl and let cool to room temperature. (Any leftover pickled vegetables can be stored with their brine in the refrigerator in an airtight container for up to 6 weeks.)
Make the tenderloin: Heat the oil in a dutch oven over high heat. When hot, sprinkle the garlic powder, Chinese five spice, and salt over the pork tenderloin, and add to the pot. Sear the tenderloin for 2 minutes per side, then reduce heat to low, cover, and cook 15-20 minutes more, flipping the tenderloin after 10 minutes. (This should yield an internal temperature of 145F.) Transfer the tenderloin to a cutting board and rest 10 minutes before slicing thinly against the grain.
While the pork rests, heat the oven to 350F. Warm the bread and crisp up the crust.
To assemble, layer soy sauce and mayonnaise on both sides of the bread, followed by jalapeños, sliced pork, pickled vegetables, cucumber slices, sriracha, and cilantro. Enjoy warm!