Remove stems and discard any grapes that aren't firm and unblemished. Place grapes in a quart-size mason jar and set aside.
Combine vinegar, water, sugar, red onion, mustard seeds, peppercorns, cinnamon stick, bay leaf, star anise pod, and allspice in a saucepan; bring to a boil. Reduce heat to low and simmer until onion is softened, about 10 minutes. Remove saucepan from heat and cool for 15 minutes.
Carefully pour cooled liquid over grapes and gently swirl jar to incorporate spices. Cover jar and refrigerate 1 day before eating.