Plumbing

Ok, so major issues with drainage after clearing the mains and all lines and all vents cleared. Now all of sudden its backing up again. After dealing with water in basement constantly " no back water valve " I noticed the out side main plumbing cast iron city vent is missing it's top. I saw a kid throw something down it and I went looked and researched and it's a vent. This is probably the issue with it now. Who do I contact to replace this cap and who is responsible for this. It's not cheap having rescue rooter here multiple times. Cost over 2000$. Please help!