Add water to a large skillet, filling to a depth of 1 inch, and bring to a boil. Add chicken, and simmer 8 minutes or until done. Cool, and cut into thin strips.
Combine chicken, tomato, onion, cilantro, and jalapeño in a large bowl. Combine vinegar, salt, oil, and black pepper. Add vinegar mixture to chicken mixture; toss gently to coat. Add avocado just before serving; toss gently to coat. Serve chicken mixture over lettuce leaves.