Sounds like you possibly have a birds nest in the vent. You are most likely going to need to call out a plumber to auger it out. In addition, you said the drains were rotted away in the bar. You need to be sure that a) all the traps are filled with water to block sewer gas from escaping into the building and/or b) if the drainage piping hasnt been repaired yet, you need to block off any open piping with rags, duct tape, etc. Additionally, drains in bars are the foulest smelling things my nose has run across if not properly maintained(speaking only for myself of course....I'd rather smell a septic tank than a bar drain). The vent doesnt eliminate sewer gases, but provides air into the system. Hope this helps you out and good luck to your daughters in their business venture.