Add the vinegar, chicken stock and thyme to the skillet and simmer, scraping up the browned bits from the bottom of the pan. Reduce the heat to low and cook, turning the chicken once, until white throughout, about 7 minutes. Transfer the chicken to warmed plates. Simmer the sauce over moderate heat until slightly reduced and flavorful, about 1 minute. Season the sauce with salt and pepper, pour it over the chicken breasts and serve.