In a large pot, combine the white vinegar, water, salt, sugar, black peppercorns, red pepper flakes, and garlic powder. Bring to a boil over medium heat, stirring until the salt and sugar dissolve. Remove from heat and let cool slightly.

Add the shrimp to the pot and ensure they are fully submerged in the brine. Cover and refrigerate for 1 hour.

After 1 hour, drain the shrimp and transfer them to a large bowl.

In a separate bowl, whisk together the lemon juice, olive oil, red onion, celery, green bell pepper, and parsley. Pour this mixture over the shrimp and toss to combine.

Cover the bowl and refrigerate for at least 4 hours or overnight for best flavor.

Serve chilled as an appetizer or over a bed of greens.