Marinate the pork with lemongrass, fish sauce, brown sugar, garlic, and shallot. 20 minutes on the counter while you prep everything else, or overnight if you are organized.
Toss the carrot and cucumber with rice vinegar, sugar, and a pinch of salt. Set aside; they pickle while you cook.
Heat the oil in a wide pan over high heat until it shimmers. Lay the pork in a single layer and sear 2 to 3 minutes per side until the edges caramelize. Work in two batches; a crowded pan steams.
Build the bowls: rice, pork, drained pickles. Herbs on top if eating now, packed separately if prepping.