Before the employee is fully vested, the method through which workers acquire a particular portion of irrevocable rights over company contributions made to the worker"s retirement plan account annually. An employee can be vested with the added 20% in each subsequent year, in at least 20% of their accrued benefits following a preliminary amount of service until full vesting occurs. The first period of service can change according to the way in which the company establishes the level of its own contributions.