1/2cucumber, medium, peeled and thinly sliced, for serving
To make the pickled vegetables:
Whisk the rice vinegar, sugar, and salt together in a medium bowl. Add the carrots and daikon and mix. Let stand for at least 30 minutes or up to 2 hours. For longer storage, transfer the vegetables to a covered container and refrigerate for up to 2 days.
To make the meatball mixture:
Combine the ground pork, shallots, fish sauce, cilantro, cornstarch, garlic, salt, and pepper together in a medium bowl. Cover and refrigerate for at least 15 minutes or up to 4 hours.
To make the caramel sauce:
Combine the sugar and 2 tablespoons water in a medium saucepan. Cook over medium-high heat, stirring constantly, until the sugar dissolves. Cook without stirring, occasionally swirling the saucepan by its handle to combine the syrup and washing down any crystals on the inside of the saucepan with a pastry brush dipped in cold water, until the mixture caramelizes to the color of an old penny, about 3 minutes.
It should be smoking and smell slightly acrid. Carefully add 1 cup hot tap water (it will splatter and harden) and stir until the caramel is dissolved. Remove from the heat.
Making the Meatballs and the Sandwich:
Using your wet hands rinsed under cold water, shape the pork mixture into 16 equal meatballs. Transfer to a baking sheet. Heat the oil in a skillet over medium-high heat. Add the meatballs and cook, turning occasionally, until lightly browned, about 6 minutes. Add the caramel sauce.
Cook, uncovered, until the meatballs are cooked through and the sauce has reduced to about 3 tablespoons, about 6 minutes. Stir in the fish sauce and season with hot sauce. Remove from the heat and cover to keep warm.
For each sandwich, place 4 meatballs in a roll and drizzle with some of the caramel sauce. Drain the pickled vegetables. Top the meatballs with as much of the pickled vegetables as you wish, along with the cilantro leaves and cucumber slices.
Serve warm, with the hot sauce and any remaining pickled vegetables on the side.