Slice the cucumbers and red onion thinly using a mandoline or sharp knife.
In a small bowl, whisk together the white wine vinegar, olive oil, sugar, salt, and pepper until the sugar dissolves.
Place the sliced cucumbers, red onion, and chopped dill in a large mixing bowl.
Pour the dressing over the cucumber mixture and toss gently to coat everything evenly.
Cover the bowl with plastic wrap or transfer to an airtight container.
Refrigerate for at least 30 minutes, or up to 4 hours, to allow the flavors to meld.
Stir the salad well before serving to redistribute the dressing.