Prepare the Quick Pickles: In a small bowl, combine the shredded carrots, radishes, diced jalapeño, 2 teaspoons of rice vinegar, a pinch of sugar, and a small amount of kosher salt. Mix well and let the vegetables sit to lightly pickle while you prepare the shrimp mixture.
Make the Shrimp Mixture: Add 8 cilantro sprigs, garlic, ginger, lemongrass, chili, 2 teaspoons of fish sauce, and the remaining 2 teaspoons of rice vinegar to a food processor. Blend the ingredients until finely chopped. Add the shrimp and pulse the mixture until it forms a coarse paste, scraping down the sides as needed.
Cook the Shrimp Cakes: Heat the sesame oil in a large skillet over high heat. Scoop the shrimp paste into the pan in portions and flatten slightly to form small cakes. Cook for about 2 to 3 minutes on each side, or until the cakes are golden brown. Sprinkle lightly with salt as they finish cooking.
Assemble the Sandwiches: Spread mayonnaise and Sriracha onto the cut sides of the toasted baguette pieces. Layer the shrimp cakes on the bottom half, then top with the quick-pickled vegetables, the remaining cilantro sprigs, and thin slices of jalapeño. Cover with the other half of the baguette to complete the sandwiches.
Serve: Serve the shrimp banh mi immediately for the best flavor and texture.