1. Combine the lime juice, vinegar, and sugar in a medium saucepan (not aluminum or cast-iron) and bring it to a boil. Boil until reduced to one cup. Let cool slightly, transfer to a blender, add cilantro, and blend until smooth. Season to taste with salt and pepper. ( The Glaze can be made a few days in advance, covered, and kept in the refrigerated. Bring to room temperature before serving.).
2. Heat the grill to high. Set aside a few teaspoons of the glaze to use after cooking.
3. Spear one shrimp onto each skewer, pushing the skewer through booth ends of the shrimp to help it lie flat on the grill. Brush the shrimp with oil and season with salt and pepper.
4. Grill the shrimp, brushing often with the glaze, until pink, opaque, and just cooked through, 1 1/2 to 2 minutes on each side. Remove the shrimp to a platter and brush with the reserved glaze. Sprinkle with cilantro and serve immediately.