Complex sentence: While still young he probably studied with Vanhal, and from 1772 he became the pupil of Joseph Haydn in Eisenstadt; evidently he had a close relationship with his teacher, and Haydn considered him to be a superb student.
Split into simpler sentences: While still young he probably studied with Vanhal, and from 1772 he became the pupil of Joseph Haydn in Eisenstadt. Pleyel evidently he had a close relationship with Haydn, who considered him to be a superb student.