In blender, puree tomatillos, cucumber, celery, cilantro, jalapeño, and lime juice until very smooth, about 1 minute. Strain through medium-mesh sieve into pitcher, pressing on solids to extract as much juice as possible.  Whisk in horseradish, pepperoncini brine, hot sauce, 1/4 tsp pepper, and pinch salt. Stir in vodka and serve over ice with pepperoncini and red chili flakes if desired.