(b) A member shall be fully vested in the employer contributions made on that member's behalf, and related earnings, after five years of service. A member is partially vested in the employer contributions made on that member's behalf, and the related earnings, in the ratio of

1. (1) 25 percent with two years of service;
2. (2) 50 percent with three years of service; and
3. (3) 75 percent with four years of service.