You are vested when you have the equivalent of five years of service credit. If you have established reciprocity with another qualified retirement system, your reciprocal service credit is included in determining eligibility for vesting and retirement.
Being vested means that you have earned the right to a future retirement benefit, even if you leave your job before you are eligible for a Service Retirement and become a Deferred-Vested member. Being vested also means you are eligible for a Non-Service Connected Disability Retirement.