Soak the lemongrass or skewers in warm water for 30 mins.
In a medium bowl, whisk together the light soy sauce, brown sugar, ginger, rice vinegar, lime juice, sesame oil, 2 tablespoons of grape- seed oil, toasted sesame seeds and thinly sliced scallions. Set aside.
Season the shrimp well with kosher salt & fresh ground pepper.
Thread them on the skewers, 3-4 on each. If using the lemongrass, you will need to skewer the shrimp with a wooden skewer first to create the holes, then thread onto the lemongrass.
Prepare your grill, bringing to medium high heat.
Brush the shrimp lightly with oil and grill for about 2 minutes per side, just until opaque and pink throughout. Remember, they will continue cooking a bit after they are removed.
Place on a platter and drizzle well with the sesame scallion sauce and garnish with ½” sliced scallions. Serve with the additional sauce.