A vesting option is an optional year at the end of the contract that becomes guaranteed if the player reaches a certain performance incentive threshold. Vesting options are typically based on playing time incentives such as plate appearances, innings pitched, games started or games finished. In most cases, a vesting option that fails to vest can still be exercised as a club option.
Santiago Casilla played the final guaranteed season of a three-year contract with the Giants in 2015, but he had a vesting option for the '16 season that went into effect when he reached 55 games finished. Casilla finished exactly 55 games, automatically locking in his salary for '16.