Pour vinegar into a pot over high heat. Stir in lemon slices, jalapeno peppers, garlic, onion flakes, and sugar. Bring to a rapid boil, stirring until sugar is dissolved. Remove from heat, and cool to lukewarm.
Place the shrimp and cilantro into a resealable plastic bag. Pour the lukewarm sauce into the bag, seal tightly; refrigerate overnight, turning the bag over several times.
Drain liquid from resealable bag, and place shrimp in a large bowl. Season with salt and pepper, and toss well.
