In a medium bowl or jar, combine mayonnaise, white vinegar, garlic powder, black pepper, salt, and sugar. Whisk or shake vigorously until smooth, about 1-2 minutes.

Wash and dry cucumbers, tomatoes, red onion, and red bell pepper. Chop cucumbers, dice tomatoes, slice the onion, and roughly chop the bell pepper.

In a large mixing bowl, add chopped cucumbers, tomatoes, red onion, and red bell pepper. Toss gently to distribute the ingredients.

Pour the prepared dressing over the vegetables. Toss everything together until evenly coated.

Serve immediately or refrigerate for up to 4 hours; store the dressing separately if making in advance.