In a medium bowl combine tomato sauce, vinegar, garlic, chili powder, cumin, oregano, sugar, salt, and pepper. Heat the oil in a large skillet over medium-high heat.
Add the chicken breasts and cook for 3 minutes on one side, flip and cook for 3 minutes on the other side. Add the tomato sauce mixture and bring to a boil. Reduce heat and simmer, covered, for about 20 minutes or until the chicken is cooked through (the internal temperature has reached 165 degrees F).
Transfer the chicken to a cutting board and shred using two forks. Return the chicken to the pan with the sauce and continue to cook for about 5 more minutes.
To make the pico de gallo- In a bowl combine plum tomatoes, onion, peppers and cilantro. Add the lime juice and salt. Mix well. Cover and refrigerate for 1 hour.
To prepare tostada shells- Pre-heat the oven to 375 degrees F. Spray tortillas on both sides with cooking spray and place onto a baking sheet.
Place into the oven and bake turning once for 10 minutes or until crispy and light brown.
Spread a layer of guacamole on each tostada shell, followed by a layer to refried beans. Add some of the shredded chicken and top with lettuce, cheese, and pico de gallo.