Yeah, that's the conspiracy theory sticker.
The funny part is that there are a lot more MIA soldiers from WW2 than 'Nam, Where are the Germans and Japanese hiding them?
As Kit mentioned, it's drifted back to a more general meaning, as in support for those who were POW and MIA.
The whole thing originally stemmed from the fact that Vietnam was the most brutal war and different war that we had fought. It was in a terrain that we had almost no way to train for and was against people who were literally willing to do anything to win. That doesn't even take into account the fact that many of those people saw us as interlopers trying to take their homes.
It was also the first conflict that we, as a country, could point to as a clear loss. Therefore, many people believed that we left quite a few people behind to rot in POW camps. My dad was in Nam, so he got pretty into the movement in the 80s. He did admit that there was a large amount of conspiracy theory behind it, but he also said that you couldn't rule out the fact that there may be people still there. Also, it was pretty huge with biker culture. We used to go to bike and skydiving rallies and there was almost always a POW speaker (or at least major presence) and literature.