Thousands of new in the box Zippo lighters were left behind when the U.S. military left Vietnam. In the early 90's Vets started returning to Vietnam. A huge demand for old Zippo lighters was created, in only a few years the large stockpile of New Zippos were
engraved and aged to look like the Zippos of old.

Since then the market has grown with new present day Zippos being engraved to reflect the Vietnam War.

There are many web sites to help determine the fake from the real, one of the better sites is   

http://perso.orange.fr/zippo.vietnam/fake.htm  (cut and paste this link to your browser)