So you want a little piece of memorabilia from Vietnam? Fine, who better to supply it than from someone who actually lives there?
One of the most popular items for sale seems to be the ubiqitous Zippo lighter, favoured by US troops the world over. However, the market is such that, there are probably more of these things in circulation than there ever were guys over there. The reason? Nostalgia sells. If you were to visit Vietnam now, you would find thousands of these lighters for sale. (I have, and they're everywhere.) Not a problem really, as everyone has to make a buck. But just be aware that the likelyhood of you buying the genuine article is pretty minimal.
How can you tell? Well, it's not easy. Zippos haven't changed much if at all over the years, so it's a simple design to copy. Batter it a bit, scratch on a badge and an inscription, and it looks pretty authentic. However! Have a look at the sellers feedback. Are they selling one or two, or have they what looks like thousands in stock? Ask yourself where they can get vast quantities after all this time (The war ended over 30 years ago, after all.)
I'm not saying DON'T buy. In fact, why not, as it's one way of these guys actually managing to earn a decent crust. But don't kid yourself on that you've bought a lighter from a dead GI.
I hope this helps.