Toss the onion with the vinegar and 2 tablespoons water in a small bowl. Let stand 15 minutes, then drain. Meanwhile, cook the turkey bacon as the label directs until crisp.
Mash the avocado with the yogurt in another small bowl until smooth. Season with salt and pepper.
Lay 4 slices of bread on a cutting board and spread with half of the avocado-yogurt mixture. Top with the lettuce and tomato and season with salt and pepper. Add a layer of turkey breast, bacon, cucumber and red onion. Spread the remaining avocado mixture on the remaining 4 bread slices and place spread-side down on top of the sandwiches. Cut each sandwich in half to serve.