Bring a large pot of salted water to a boil. Add shrimp and cook just until pink and opaque, about 2-3 minutes. Drain immediately and cool completely.
In a saucepan, combine white vinegar, apple cider vinegar, water, olive oil, salt, sugar, celery seed, peppercorns, garlic and crushed red pepper flakes. Bring to a gentle simmer for 2-3 minutes, stirring until salt and sugar dissolve. Remove from heat and let cool slightly.
In a large glass container or airtight dish, layer shrimp, onions, hot peppers and lemon slices. Pour the brine mixture over the top until everything is submerged.
Cover and refrigerate at least overnight, preferably 24 hours, allowing the flavors to fully develop. Stir once or twice while marinating if possible. Serve it with saltines, cocktail picks, cold beer and a porch breeze.