One grilled flank steak, thinly sliced and skewered or made into banh mi sandwiches, is a delicious, economical, efficient way to feed a crowd. Guests can embellish their sandwiches with some vegetables from the summer-roll platter.
Preheat a grill or grill pan to medium-high heat. Rub steak with oil and season with salt and pepper; let stand 10 minutes.
Grill steak, flipping once, until charred on both sides and a thermometer inserted into thickest part of steak reaches 125 degrees for medium-rare, about 6 minutes. Let steak rest at least 20 minutes and up to 1 hour. Thinly slice steak against the grain and thread onto bamboo skewers. Serve with nuoc cham sauce, bread, herbs, and vegetables.