In a small bowl, whisk together the white vinegar, sugar, water, salt, and black pepper until the sugar and salt are fully dissolved. Set aside.
Place the sliced cucumbers and onions in a large bowl. If you have time, sprinkle them with a pinch of extra salt and let sit for 10 minutes, then pat dry with paper towels to remove excess moisture (this helps prevent a watery salad).
Pour the dressing over the cucumbers and onions. Toss well to coat everything evenly.
Cover the bowl and refrigerate for at least 30 minutes to allow the flavors to meld. The longer it rests, the better it gets.
Before serving, give the salad a stir. Taste and adjust seasoning if needed—add more sugar for sweetness or more vinegar for tang. Garnish with fresh dill or parsley if desired. Serve cold.