Put the noodles in a large bowl and cover with warm water. Soak for 10 minutes.

While the noodles are soaking, bring a large pot of salted water to boil. Prepare a bowl of ice water.

Drain the noodles from the warm water and place in boiling water on the stove. Boil the noodles carefully until just cooked (al dente) (the water should have just returned to a boil after you place them in the pot). Drain the noodles again and dunk them into the ice water to stop them from cooking, then drain again. Place the noodles on a platter and fluff with chopsticks. Set aside to cool completely.

Put the cucumber, carrots and bean sprouts (if using) in a colander and add 2 tsp. sugar, vinegar and a pinch of salt and mix well. Allow the vegetables to absorb the vinegar mixture, then drain well and blot with paper towels to absorb excess liquid.

Put the pork in a bowl. Add 2 tsp. sugar, fish sauce, scallions and 1/4 tsp. salt and 1/4 tsp. pepper. Mix the pork well so that the scallions are evenly distributed, then form the pork into small patties approximately 2 inches wide, so that you have 18-20 patties. Place on a platter and cover loosely with a damp paper towel.

Heat a grill or pan over medium-high heat. Sear or grill patties in batches, approximately 2-3 minutes per side, until meat is cooked through and no longer pink. Remove from pan or grill and place on a paper towel-lined plate.

Divide the rice noodles into four bowls. Divide the vegetables and pork patties evenly, then add lettuce. Top with cilantro and mint.

Mix together the ingredients for the dipping sauce, adding more or less water to taste (the sauce should be tangy and fragrant, but not too sour).

Serve dipping sauce with noodle salad.