The plan in question is a DB plan that was frozen with respect to benefit accruals, new participants. In addition, the plan states that rehires cannot resume participation under the plan. However, if a participant left employment and was rehired before incurring 5 break in service years, the plan would have to give the participant credit for past service, but would the plan have to let the rehire continue to earn vesting service upon rehire (even if the person wasn't eligible to participate in the plan going forward?). The plan allows those who continue their employment to continue to accrue service for purposes of vesting.

In general, ERISA definitions of vesting and vesting service focus on the employment relationship, not the plan relationship. I believe vesting service cannot be frozen.