Legal Question in Real Estate Law in Illinois

Most likely the tenant but Illinois is what's called a "comparative negligence" state. That means each party could be liable if each was negligent. That's a question of fact. In your case my primary questions to help determine this would be (a) level of intoxication, (b) code compliance of the stairway and top and bottom landings, (c) condition of the stairway and top and bottom landings; (d) was the vase an unreasonable obstruction at the bottom landing. If the tenant required hospitalization and had any kind of health insurance (generally renter's insurance doesn't cover this and you don't say whether the tenant had any), the tenant may report this as "3rd party liability" meaning somebody's insurance (other than the tenant's) could be responsible.