In a nonreactive stockpot over medium-high heat, bring the vinegar, water, sugar, salt, peppercorns and bay leaf (and chile flakes, if using) to a boil, stirring until the sugar dissolves. Add the cucumbers (or pineapple), then remove from the heat and let cool completely. Transfer to an airtight container and refrigerate until serving. Drain before serving. (Pickles keep, refrigerated, about 2 weeks.)