Place chicken in a medium saucepan; add chicken stock to cover. Bring to a boil. Reduce heat to medium, and cook, uncovered, 15 minutes or until chicken is done.
Remove from liquid. Let chicken cool to touch. Chop chicken into bite-size pieces.
Combine chicken, sprouts, water chestnuts,green onions, and garlic. Combine vinegar, soy sauce, and oil; pour over chicken mixture, tossing gently.
Line each pita half with 1 lettuce leaf; spoon chicken mixture evenly into pita halves.
1⁄2 cup bean sprouts
1⁄4 cup diced water chestnut
1⁄4 cup sliced green onion
1 tablespoon rice vinegar
1 tablespoon minced garlic