In a bowl, whisk together white vinegar, water, sugar, salt, and pepper until sugar is dissolved. Stir in dill and red pepper flakes if using.

With a sharp knife or mandoline, cut cucumbers and red onion into uniform, thin slices for even marinating.

Transfer cucumber slices and onion into a large bowl. Pour the vinaigrette over the top and gently toss to coat every piece.

Cover the bowl with plastic wrap and refrigerate for at least 8 hours (or overnight) to let flavors meld.

Give the salad a final stir, then transfer to a serving dish. Garnish with fresh dill or a sprinkle of black pepper.