Complex sentence: Schubert never forgot the experience, and determined to write for Vogl; and when the two finally met, in 1817, Vogl was as impressed with the quality of Schubert's music as Schubert was with Vogl's singing.
Split into simpler sentences: Schubert never forgot the experience, and determined to write for Vogl. When composer and singer finally met, in 1817, Vogl was as impressed with the quality of Schubert's music as Schubert was with Vogl's singing.