He can't take away benefits you already earned or have a contact for. So, depending upon how vacation is calculated, you may have already earned it. However going forward, benefits can be reduced. You can work for another employer.
If the employer tries to use a change in policy to retroactively take away vacation time that has already been earned and accrued, then you may have a claim against the employer under the Illinois Wage Payment and Collection Act. However, if the employer has only changed the policy with respect to vacation time going forward, then it is unlikely that you would have a claim.