Thinly slice the cucumbers and red onion using a mandoline or sharp knife. Place them in a large mixing bowl.
In a separate small bowl, whisk together the white vinegar, sugar, salt, and black pepper until the sugar dissolves completely.
Pour the dressing over the sliced cucumbers and onion. Add the chopped dill.
Toss everything together until evenly coated. Make sure all slices are well covered with the dressing.
Cover the bowl and refrigerate for at least 30 minutes, or up to 2 hours for best flavor development.
Before serving, give the salad a final toss to redistribute the dressing. Taste and adjust seasoning if needed.