In small saucepan stir together vinegar, sugar, seeds, and peppercorns and bring to a boil. Remove from heat and stir to dissolve sugar; let cool to warm. Stir in grapes and onion and set aside to cool and marinate for 1 hour, stirring occasionally. Grill hot dogs over medium heat for 5 minutes, turning occasionally, or until nicely charred and heated through. Place hot dogs in buns. Using a slotted spoon, top with pickled grapes. Makes 8 servings.