Spiced (and Spiked) Concord Grape Punch

Combine grape juice and spices in a large pot. Bring just to a simmer, then remove from heat and let stand, covered, 20 minutes. Strain through a fine sieve (discard solids), and chill. To serve, combine with simple syrup and vodka, if using. Serve over ice in a tall glass, and garnish with grapes.