Peel and slice cucumbers. Deseed, if desired. Coarsely chop into bite-size pieces. Slice and chop tomatoes. Place cucumbers and tomatoes in large bowl with chopped onions, if desired.
To make dressing: In separate bowl, mix apple cider vinegar, olive oil, sugar, onion powder, garlic powder, dill and mayo. Season with salt and pepper, to taste. Pour over cucumber mixture and mix well.