Good question! I spent too much time researching this topic...
The "plastic" you are referring to breaks down very easy when exposed to the elements. In addition, the water permeability of this stuff is quite high.
A true vapor retarder (the correct term for this stuff) has passed certain standards setup by ASTM (American Society for Testing and Materials):
(taken from another website):
"ASTM E 1745–97 “Standard Specification for Plastic Water Vapor Retarders Used in
Contact with Soil or Granular Fill under Concrete Slabs.”. is a standard that was
written by a committee of ASTM chaired by Tom **** an architect in Northern
California.
Three performance classes are outlined A, B and C (with Class A being the best).
Permeance levels are the same for each class. Tensile Strength and Puncture
Resistance change with each class.
1745 refers to ASTM E 154-93 “Standard Test Methods for Water Vapor Retarders
Used in Contact with Earth Under Concrete Slabs, On Walls or as Ground Cover.” and
to ASTM E 96-95 “Standard Test Methods for Water Vapor Transmission of Materials."
In addition, these standardized materials are more robust and do not break down in nearly as fast as normal "plastic" (if at all).
