I fractured my leg just above the ankle in April. When I applied for short-term disability, I was informed I would have to use my vacation time first, before I would get the disability insurance. The home office is based in MO., and I know that Mo. has a law that states you must use accumlated vacation time first. I would like to know if Illinois has the same law, or if it is up to the employer's discrimination to use vacation time.

I don't believe Missouri has any such law. In both instances this would be a matter of the employer's policy and the fact that the federal Family and Medical Leave Act allows the employer to require an employee to use vacation/PTO when he takes FMLA leave. If your employer has such a policy, then any disability benefit they provide would start after all vacation has been paid out and any waiting period in the disability plan is satisfied.