Trim crusts from bread slices; cut slices into triangles.
Heat a small nonstick skillet over medium-high heat. Coat pan with cooking spray. Add onion to pan; sauté 5 minutes. Combine sugar and 1 1/2 teaspoons vinegar, stirring with a whisk until sugar dissolves. Add sugar mixture to pan, scraping pan to loosed browned bits. Remove from heat; cool to room temperature.
Combine onion mixture, remaining 1 tablespoon vinegar, chicken, and remaining ingredients. Place about 2 tablespoons chicken mixture on each of 12 bread slices. Top with remaining bread slices.