Employers are permitted to take away vacation days which an employee has already earned. This is often done as a punishment instead of termination of the employee.
According to the NOLO legal website, employers are not required by law to offer vacation days, and as a result, they are permitted to set their own rules regarding the vacation days they offer. This means that an employer can legally choose to revoke vacation days without offering the employee a reason. In most cases, a company's employee handbook outlines everything an employee needs to know about vacation days, including for what reasons they may be revoked.