Benefits are not contract items for at will employees, if that is what you mean. If you resign, and do not negotiate any severance package, then I am afraid the horse has already left the barn.
Before you resigned, you should have negotiated a seperation agreement to include a pro rata accounting for the unused vacation time. Now the company can and will say you waived vacation by your resignation.
Employers are not required to give paid vacation, but if you had vacation that was available to you to use on your last day of employment had you not left the company (and it sounds as if you did), you are entitled to be paid for your unused vacation. The Illinois Wage Payment and Collection Act specifies that an employee who leaves without having taken all vacation time (as well as personal holidays) must be paid the dollar equivalent of the vacation time.
You should contact the IL Dept. of Labor to file a wage claim.
With regard to the FMLA denial, I'm curious as to why you say you can't prove it. I'd recommend a consultation with an employment attorney to discuss your situation.
(Note: This answer is not intended to create an attorney-client relationship.)