Large Italian or Thai basil leaves, or shiso, sometimes called Japanese mint, can be used in place of the traditional, but obscure, pepper leaf.
Soak bamboo skewers in water for 30 minutes. Trim lemongrass stalks to 5 inches from base; remove bad outer leaves; cut into 1/2-inch lengths. In a food processor, process lemongrass to a hairlike texture. Transfer to a bowl; add garlic, fish sauce, soy sauce, sugar, turmeric, and oil.
Slice beef 1/4 inch thick, then 2 inches long; add to marinade; toss to coat. Marinate, refrigerated, 30 minutes to 8 hours.
Fold steak slices in half; thread 5 slices onto each skewer, wrapping every other slice with a pepper leaf before skewering.
Add vermicelli and salt to boiling water; cook until al dente, about 3 minutes. Drain and rinse; while wet, use a fork to twirl about 1/3 cup noodles against side of bowl to make a nest 3 inches wide. Transfer to a serving dish. Repeat until all noodles have been used. Drizzle nests with Scallion Oil.
Brush a grill or grill pan with oil; heat until hot. Grill skewers until meat is well charred, 4 to 5 minutes for medium rare. Arrange skewers around noodle nests; serve immediately. Top with Vietnamese Dipping Sauce, peanuts, and scallion greens.