Having been all over Vietnam and having done lots of traveling of various sorts, it's really going to depend on you and your shooting style, and your travel style for that matter. For Vietnam in general I would not even take a prime lens.
With the K-3 I would take something like the 16-85mm. And that's it. I personally did a couple of around the world trips with that lens and a K-3. When I say that lens, I mean ONLY that lens. Depending on how you travel you should take just what you will need and something very flexible. In Vietnam especially WR lenses IMO are a complete must. It's hot and humid and depending on when you go, you can experience serious rain.
If I was going to fly off tomorrow to Vietnam I would take my K-1 and the 24-70. No tripod, no fluff, just a few extra batteries and a couple of chargers. I personally use the standard charger body that comes with the camera (I usually take two) and I also take a variety of plugs. Also if I was going to Vietnam I would definitely take a dry bag as well. One big enough to hold the camera is fine but up to you.
If you look at the pictures on this listing on ebay you can get all sorts of plugs like that that have no cables to fumble with. It's a plug head that plugs directly into the charger. I don't know the technical name. You can get them straight or bent to 90 degrees. The 90 degree ones work great for anything in the wall.
Battery Charger For Konica Minolta NP-400 DiMAGE A1 A2 / Pentax D-LI50 K10 K10D | eBay
I usually carry two of each. They are small, very light, and don't take up hardly any space. I tend to bounce around a lot so I take more variety. If you will just go to one country take plugs for that country.
For lenses, particularly in Vietnam, a zoom is a must. The action does not stop to allow you to swap lenses or whatever. Unless you have a specific mission for a specific type of photo no real need to haul around any primes. This is assuming you are just going to have fun and document things as you go.
As for long lenses, it's just my experience but they are of limited use in a lot of travel scenarios. Naturally you can take photos with whatever you have, but I tried hauling around a 70-200 on an around the world trip and ended up packing it up and mailing it back home.
Again it's all going to boil down to your personal travel habits and style.
The only real downside to many of the zooms (personal opinion) is they are not standard aperture. I loved the 16-85, but I love the 24-70 even better, especially on the K-1. When it starts getting into low light photography there is definitely a difference. In the day time I prefer to be able to set my aperture and not have it shift on me as with the variable aperture ones. Just personal preference there .