There are a few different ways to become a vested IPERS member.
Regular members
As of July 1, 2012, regular members become vested after accruing seven years (28 quarters) of service, or are at least 65-years-old while working in IPERS-covered-employment, whichever occurs first.
Prior to July 2012, members vested with four years (16 quarters) of service or when they were at least age 55 working in IPERS-covered employment, whichever happened first. If you vested before July 1, 2012, you remain vested.
Special service members
You vest once you've accrued four years (16 quarters) of service or are at least 55-years-old while working in covered employment, whichever happens first.