Stir together lemon grass, 1/4 cup ginger, garlic, cilantro, 1 minced chile, and 3/4 cup peanut oil in a large bowl. Add the shrimp, and toss to coat; let marinate at room temperature for 20 to 30 minutes.
Preheat a grill for medium-high heat.
Meanwhile, pour lime juice, rice vinegar, mirin, soy sauce, and water into the bowl of a blender or food processor. Add the lime zest, 1 tablespoon ginger, fish sauce, 2 chile peppers, garlic, and peanut butter; process until smooth. While processing, slowly pour in the peanut oil; process until smooth and creamy. Pour into a bowl, stir in mint, cilantro and chopped peanuts; season to taste with salt.
Remove shrimp from marinade, shaking off any excess. Grill on preheated grill until pink and firm, about 2 minutes per side. Serve immediately with sauce.