Complex sentence: The band had started writing and recording when Jimmy Bain was fired and replaced by Teddy Cook and Vinny Appice was fired and replaced by former AC-DC drummer Simon Wright.
Split into simpler sentences: The band had started writing and recording when Jimmy Bain and Vinny Appice were fired from the band. They were replaced Teddy Cook and Vinny Appice was fired and former AC-DC drummer Simon Wright.