1. In a bowl, whisk together tomato juice, vodka, lemon juice, Worcestershire, salt, horseradish, and celery seed. Add flank steak and red onions to marinade and chill at least 4 hours and up to 16 hours.
2. Prepare a hot charcoal or gas grill. Grill steak and onions, turning once and basting halfway through cooking, until onions are browned (6 to 8 minutes) and meat is done the way you like it, 10 to 12 minutes for medium-rare (cut to test).