when does employer have the right to change policy of vacation pay, without notifying the employee?And does employer have to have employee sign off?

First, there is no "law" which requires an employer to give employees vacation, paid or unpaid.  Vacation is a matter of either an agreement between the company and the employee or a company policy.  If the vacation is part of a written agreement or contract between the company and the employee, the company cannot change the terms without the consent of the employee.  However, if the vacation plan is simply a company policy, the company has the right to change the policy unilaterally without any notice to the employee. But, if the company has a policy that allows employees to take paid vacation, and the employee has earned the vacation but not taken it (called "accrued vacation")  the company may owe the employee compensation for the vacation the employee hasn't taken if the company changes the policy.  Stated differently, it may be a violation of the law for a company to tell a worker that it has changed its vacation policy without notice and will no longer pay for vacation the employee has already earned but not taken.