A qualified participant is immediately 100% vested in his or her contributions made to Tier 2. A qualified participant shall vest in the employer contributions made on his or her behalf to Tier 2 according to the following schedule:

  (a) Upon completion of 2 years of service, 50%.

  (b) Upon completion of 3 years of service, 75%.

  (c) Upon completion of 4 years of service, 100%.