Prepare vegetables: Dice cucumbers evenly for uniform marination. Slice tomatoes into wedges (keep skins intact for texture). Thinly cut onion to avoid overpowering the salad. Place all veggies in a large bowl.
Make the marinade: In a mason jar, combine vinegar, water, oil, sugar, salt, pepper. Shake vigorously for 10–15 seconds to dissolve sugar and salt.
Combine with veggies: Pour marinade over sliced vegetables. Add basil strips and stir until all pieces are evenly coated. Use a spatula to gently fold components if you want to preserve tomato texture.
Marinate and refrigerate: Cover bowl with plastic wrap. Chilling for at least 2 hours is essential to allow flavors to develop fully. For stronger taste, let sit overnight.
Drain before serving (optional): Some recipes suggest briefly draining excess fluid for a crisper texture.