For the dipping sauce: Combine all of the ingredients in a small bowl. Stir together and set aside.
For the lettuce cups: Heat the sesame oil in a large nonstick skillet over medium heat. Cook the garlic, lemongrass and shallot, stirring, until fragrant, about a minute or so. Add the shrimp and cook, stirring often, until pale pink in color, 3 to 4 minutes. Remove from the heat, stir in the chile paste, soy sauce and chopped cilantro.
Carefully break off the lettuce leaves from the core and place them on a large platter. Spoon shrimp into each lettuce cup, top with the carrots, bell peppers, cabbage and some cilantro leaves. Serve with the dipping sauce.