Sentence:  Mr Yates conceded: "These cases are very difficult to prove because they are bargains made in secret. Both parties have an absolute <WSD>vested interest</WSD> in those secrets [not] coming out." / In this context, "vested" is often used informally for emphasis. 
Meaning of the word marked with <WSD> tags:  an exceptionally strong interest in protecting or promoting something to one's own advantage