Complex sentence: The title was vacated upon a contract dispute between Penn and UFC who promptly stripped him of the welterweight title, claiming Penn had breached his contract and that the signing constituted him refusing to defend his title.
Split into simpler sentences: The title was vacated upon a contract dispute between Penn and the UFC. The UFC promptly stripped him of the welterweight title, claiming Penn breached his contract and that the signing constituted him refusing to defend his title.