Prepare the brine: Combine equal parts water and plain white vinegar. Add 25 grams kosher salt and 100 grams plain white sugar. Stir until dissolved.
Prepare cucumbers and aromatics: Slice Kirby cucumbers in half. Peel an entire head of garlic and rinse and dry. Gather fresh dill, dried bay leaves, allspice berries, mustard seeds, whole cloves, whole peppercorns, and optional red pepper flakes.
Pack the jar: Place cucumbers into a jar. Add half of the garlic cloves and a few bunches of fresh dill. Add dried bay leaves, allspice berries, mustard seeds, whole cloves, peppercorns, and red pepper flakes if using. Drop the remaining garlic cloves on top.
Add brine and seal: Fill the jar with the prepared brine, leaving about 1 cm of headroom at the top. Seal the jar tightly and shake gently to mix.
Sous vide cooking: Place the sealed jar into a preheated sous vide bath at 140°F (60°C) for 2 and a half hours.
Cool and refrigerate: Remove the jar from the sous vide bath. Wipe down the jar and allow it to cool completely to room temperature. Refrigerate overnight before serving.