That said, I'm inclined to distrust Pew's evaluations on this matter, since so many of their surveys are done by phone, and the methodology seem almost certain to cause a disproportionate focus on those people who have a landline and are either without caller id or who just go ahead and answer calls from unknown callers. This was true just a year or two ago, and I'm not really inclined to re-evaluate their methodology. Actual numbers of church attendance don't seem to reflect the trends that Pew finds, and tend to point to even more dramatic declines in observance. Obviously this is biased towards larger cities, but so is the population density so it isn't a total illusion.
The belief that shitty people will get what's coming to them through some external mechanism serves the useful function of allowing people to rationalize not engaging in settling scores and vigilante justice. I expect belief in the existence of hell to persist much longer than other "religious superstitions" (or whatever you want to call them).
I'd say it's a wash. Plenty of people who believe in hell do monstrous things, and plenty of people without this delusion behave as model citizens. Religious people constantly plead that without their belief system everyone would engage in wanton immorality, but this is just the self-reinforcing aspect of the meme.
It's not limited to Vietnam: 58% of Americans believe in Hell[1]. I'd call that widespread superstition.
[1] http://www.pewforum.org/religious-landscape-study/belief-in-...