The only thing I'd warn you about here is that you need to look at your state's law to determine whether it permits such a vesting schedule. Because governmental plans are not subject to either the vesting rules (other than vesting on plan termination) or the nondiscrimination rules of federal law, such an arrangement would not be prohibited by federal law.
Also, do make sure that you define the affected classes of employees carefully, so that there will be no question later as to who is in which group.
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