I am searching for a spreadsheet that will accrue employee vacation time and
cost balance by cost center and then reverse the accrual when the vacation is
used later, even if the employee is assigned to another contract~semi-monthly
payroll accrual period.

In essence, make sure that vacation accrual expense is accurately accounted
for as accrued, not when taken. Then when taken, the accrual will reverse
and usage will be reflected in the employees payroll balance.