In a medium bowl, whisk together lemon grass, garlic, chili sauce, fish sauce, sugar, olive oil, lime juice, pepper, and green onions. Add shrimp and toss to coat. Let marinate 15 minutes at room temperature or up to 1 hour in the refrigerator.
Prepare a gas or charcoal grill for high heat (you can hold your hand 1 to 2 in. above grill level only 2 to 3 seconds). Brush grill with vegetable oil. Thread shrimp onto skewers and grill until curled, pink, and opaque, 2 to 3 minutes per side. Remove skewers and garnish with mint.
Note: Nutritional analysis is per serving.