Complex sentence: He is best known for his Vine career that he then converted into a YouTube career when the app was shut down by Twitter at the beginning of 2017, though he had started to create content on Youtube before that as well.
Split into simpler sentences: He is best known for his Vine career until the app was shut down by Twitter at the beginning of 2017. After the shutdown of Vine, he shifted his online presence mainly to Youtube, though he had started to create content on Youtube before that as well.