A vapor barrier on the basement walls is an important part of finishing your basement.
First it will protect any water leaks from wall ties, or wall cracks (even ones that have not occurred yet).
Second, wall vapor barriers provides your finished basement a moisture barrier to help keep high humidity, condensation, and vapor transmisson under control.
Wall vapor barriers should be at least 6 mil thick.
When using a wall vapor barrier remember to use unfaced insulation so as not to trap moisture between the vapor barrier and the insulation.
The wall vapor barrier should be installed on the foundation wall and tucked into any perimeter drainage system that is installed.
Your basement walls could than be installed in front of this vapor barrier and then insulated using unfaced battings.
Over the last40 years of waterproofing basements, I have seen too many homeowners and contractors refinish a basement without installing a proper wall vapor barrier.
If you are planning on having your basement waterproofed usually the Basement Waterproofing Contractor will install a vapor barrier upon request when installing the sub floor drainage system.
If you are having your basement refinished, make sure the you contractor installs a wall vapor barrier before building your basement walls.
Hope this helps a little.
Marcel :)