Vesting Period for Annual Absence Plans

Summary:

We have a absence plan whose Accrual Method is Front loaded and repeating Period is Annual with Ceiling Rule as flat amount.

Requirement is that employee should not be able to use the accrual hours until Probation Period ends. I created a Vesting Rule Fast formula to calculate the vesting period but when i run "Calculate Accruals and Balances" as of the hire date of employee; i still see the balance in the absence records. I checked the formula is returning 120 days as vesting period which is correct. I also tried changing the vesting rule to "Elapsed Period" but still i see a balance for employee even though vesting period is not complete.