A vapor barrier is usually made of plastic sheeting set into place on the crawl space floor.  This provides a barrier between the ground and the inside of the crawlspace.  The problem with just the plastic sheeting is you are not stopping the water from coming in.  You could have plastic sheeting that is swimming in water.  Every home has different needs and sometimes it is difficult to find out what they may be.  Maybe you need a SmartSump Crawl Space Drainage System...or CleanSpace Encapsulation System...why not call the basement and crawlspace experts to find the best solution to your home.