Unless you install the SEALED vapor barrier at the time of construction, retrofitting one would be next to impossible, or simply impossible. If the house was built before the code was written, there isn't much you can do. BUT, if it was built AFTER the code was written, how did it pass inspection?! This is where you need to ctc the builder. The only other idea is to insulate with insulation with the proper 'backing'.
My best to ya and hope this helps.
PS: Don't just 'hang' a vapor barrier. If you manage to 'trap' moisture, you'll create a rotting problem.