travel preparations
Make sure you ask about the Viet Kieu discount for visas (when purchasing the plane ticket), visa extensions and travel costs (while in Viet Nam). (KL)

Let your travel agent get your visa. Asking consulates will often take longer and cost more. (KL)

money issues
Bring $100 bills with absolutely no marks on them. You can get a better exchange rate with the large bill. And most currency exchange vendors will not accept marked bills. Banks also give good rates because they have yet to learn about commission. You can withdrawal cash with your debit cards from major banks in the major cities. (KL)

social etiquette
Refrain from making “politically correct” remarks that may be critical of the Vietnamese society. Aside from just asking for trouble, making judgments about a society you’re only visiting briefly just doesn’t make sense. (KL)

Learn your honorifics. Politeness goes a long way. (KL)

Be prepared to be called a whore, slut,…if a white man is accompanying you. (KL)

bugs
Aerosol cans and bug repellent are contraband on many airlines (due to the dangers of explosion). But Avon Skin So Soft is actually (unintentionally) a fairly good mosquito repellent with a pleasant woodsy smell. Take it from me who has as my mom says "ma'u ngo.t". I spent the rainy season in Hoc Mon (i.e. the former swamp outside of Sai Gon) and it provided some relief. As for its lotion properties, weo in humid weather it doesn't really matter how soft your skin is b/c it's covered with a sheen of sweat & moisture (a natural healthy glow if you will.) On the other hand, do not bother with perfume. It attracts other bug projectiles. It's better to smell natural than be a bug magnet. Oh & Saigon smog is "heo" on your skin. Prepare to exfoliate. (Ly-Huong Nguyen)

hygiene
Bring clothes & underwear that wick away moisture... or change clothes frequently. If you go hiking or do anything active outdoors in searing sunlight where you sweat and sweat (or glisten and glisten as the case may be) when everyone else is having a sensible siesta, you will pray to the goddesses that you don't get a UTI. Bring cranberry pills & antibiotics. Drink lots of water. Otherwise there's also a tea made from some rau that clears it up fairly quickly. (Ly-Huong Nguyen)

eating out
If you eat snake meat, make sure it's well cooked. No it doesn't taste like chicken. It tastes like, weo like reptile, like frog. And yes, you are just supposed to swallow the miniscule vertabrae. You will look a fool if you try to pick them out. And it's time-consuming. Ruou tiet (snake blood rice wine) is not for the faint of heart, but it just tastes like some powerful rice wine. But I am sure the alcohol kills the germs... (I had some culinary adventures when di nhau voi may co & chu, went drinking with the aunties & uncles....) (Ly-Huong Nguyen)

As for food, you can eat at the food stalls on the street, but stick to the things being cooked on the spot, so there's no chance for bacteria acting up. Drink cold coconut milk without ice. I've applied this policy in Mexico, China and Thailand, and I was not sick for even one day. (Do Hao Nhu)

bureaucracy
Do not “tip” at the airport. If you have nothing to hide, you can be assured that you’ll get out of the airport with no problems. If you are unsure if you’ll be “taxed” for things you bring in, then a “tip” may be in order. I just don’t believe in contributing to the “tax” system at the airport. (KL)

You can carry copies of your passport when traveling. Don’t carry your passport and all your money with you everywhere. Leave it in a safe or with someone you trust when in Viet Nam. (KL)

Bureaucracy can be a pain if you have to deal with it, but it's only slightly worse than the DMV. Being polite no matter what (and not acting like an entitled American tourist) will get you greater mileage. If nothing else you don't lose face.
Oh and leave your "nhac hai ngoai" (overseas/foreign music) at home too. I had 10 of my VK CDs confiscated. Course I also didn't try to bribe. But this was a week of waiting to get all my other American CDs, and German-made VN classical music (go figure) returned, plus a fine. (Ly-Huong Nguyen)

(re-entry) visas
If you are at all planning to visit another country in the middle of your Viet Nam trip, save yourself the week of run around, bureaucracy, capriciousness & bribes. Just get a multiple entry visa while you are still safely ensconced at home. It's only a little more expensive but you save your self a lot of time and anxiety. (Try going to a VN consulate in another country and hoping that the bureaucrats on the VN side that you bribed before actually sent your paperwork in so that you can get back into VN.) If you ever have to do visa tinkering while you are in Viet Nam, go to a Cong Ty Du Lich (Travel Agency) which are licensed by the government. Not only are they more pleasant, but they'll handle your paperwork and interface with their buddies in the official Visa office. (Ly-Huong Nguyen)

appropriate attire
I think you need to remember only one thing: Do not wear shorts or "sleeveless" blouse to the pagodas, or Ho Chi Minh mausoleum in Hanoi. (CHP)

To add to the culturally appropriate attire for VN discussion, I'd like to suggest something about weather appropriate clothes.
VN has more of a diverse climate than most anticipate. If you are in the North in the Fall or Winter, it can get chilly--like San Francisco. A (waterproof) overcoat is not unheard of.
The middle region can get somewhat cool too if I remember right, but not as cold as the north.
In the South it's pretty warm throughout though a light sweater or sweatshirt is nice for the cool nights.
Of course, water proof attire for the rainy season (south) and winter (north) is a life saver--unless you plan to cab it everywhere, of course.
Mountain regions tend to be chilly all year long. Shorts may not work here. (KL)

If you are planning to visit family or go out, bring nice clothes. Vietnamese from the cities (esp. Sai Gon) are ultra trendy. You don't want to be stuck looking like a "ta^y ba lo^." (Kieu Linh)

And don't forget your bathing suit or UV protection. Viet Nam still does not sell nice bathing suits, so you should bring your own. That is unless you like wearing bright 1980s style one-pieces. (KL)

purchasing clothes
If you do choose to buy clothes in VN have it tailor-made, not at a "shop". It's the hien dai "modern" hip thing to do in Sai Gon nowadays. But tailored clothes will fit better anyways since --as in S. Korea-- there is no standardized sizing and they don't have more than one or two of any particular item (it's too expensive & risky to have too many of any one thing as they have to absorb their own costs & losses) and more importantly a ton of seamstresses have lost work. Considering that this is one of the main occupations many schoolgirls are trained for, this is creating a good deal of unemployment... which among females can lead to seeking work in cafes, bars, and other sex worker occupations. (Ly-Huong Nguyen)

Do not make clothes in Ha Noi (not the best seamstresses). In general, I think if your time is limited, just get an ao dai made. Too many people have commented that once they get back to wherever, they cannot wear any of the stuff they got tailored in Nam. (KL)

If you have at least a couple of weeks in VN, you should seriously consider making a bunch of clothes. Get your fabrics in the US (or other host country) or in Hong Kong, Thailand or wherever you may have a long stop over. VN fabrics are not of great quality and natural fabrics are difficult to find. Also have pictures or samples of the clothes you wish to make. Then have family or friends in VN recommend a good seamstress. Depending on the size of your order, you should get things back in about a few days to a few weeks. (Kieu Linh)

go moped or not?
Learn to ride a "xe may." Borrow one from family or folks you trust. And ride, baby, ride. I learned to love Sai Gon in spite of the traffic, smog, and its coarse urban-ity, when I learned to ride a "xe may" and negotiate the crazy crazy traffic. Small liberation. Big joy. Hey if a teenager can do it, so can I. (Ly-Huong Nguyen)

On Riding a moped, I was almost killed while riding on the back of my cousin's moped. No, I strongly recommend NO NO NO to mopeds. (Do Hao Nhu)

If you plan to stay for a long time and live in the city, a moped is a must. You can always resell it at the end of your trip for nearly the same price. Moped value doesn’t depreciate. (KL)