What is the origin of the idiom "vested interest" and when to use it?

The idiom "vested interest" originated from legal and financial contexts. The word "vested" refers to something that is secured or granted to someone, while "interest" refers to a stake or involvement in something. It is used to describe a personal or financial interest in a particular outcome or situation. It implies that the person has a significant stake or benefit tied to the success or failure of something.