How should I go about getting this resolved

If you feel that lot was negligent--the person had gotten access from the lot to tickets and a uniform, was standing where the valets were and acting like one, but the actual valets or supervisors weren't challenging him, etc.--you could sue the lot or the organization (the casino?) controlling it.

If the lot was not negligent, then they would not be at fault--so if someone stopped you short of the lot and said they were valet and you gave him/her the car, but the lot and it's personal had no way of knowing what this  person was doing--there would be no liability.