The Pope wanted Michelangelo 's assistant Vignola to finish it , but he was not able to . On his death the work continued under his assistant Jacopo Barozzi da Vignola with Giorgio Vasari appointed by Pope Pius V as a watchdog to make sure that Michelangelo 's plans were carried out exactly . Despite Vignola 's knowledge of Michelangelo 's intentions , little happened in this period .