In a saucepan, combine vinegar, water, sugar, and salt. Bring to a boil over medium heat, stirring until sugar and salt dissolve.
Place the sliced vegetables, garlic, mustard seeds, black peppercorns, red pepper flakes, and fresh herbs (if using) in a clean, large jar.
Pour the hot vinegar mixture over the vegetables, ensuring they are completely submerged. Close the jar with a tight-fitting lid.
Let the jar cool to room temperature, then refrigerate for at least 2 hours or overnight. The longer the vegetables pickle, the more flavorful they will become.
Serve chilled. Store in the refrigerator for up to 2 weeks.
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