I took this one cold because I've been reading about the Vietnam War since I was a kid, and it was a mistake. I was thoroughly prepared for any questions on US involvement, but the test focused mostly on the French colonial days, and the Viet Minh fight against the French. I'm a bit embarrassed to admit I didn't pass this one....it's no joke.
DSST A History of the Vietnam War I took this course back in 2002 and passed the exam by using the book "The Complete Idiot's Guide to the Vietnam War" by Timothy P. Maga.
Interactive Vietnam War Timeline There is also a vietnam war timeline that is user contributed. It has key events in the conflict and you can also add your own if you feel they should be there. Good luck!
This was a problem for many - they forgot that the Vietnam War lasted around 30 years, and not just the final 15 or so with US involvement. I seem to recall there were also some colonial questions that meant that you should really read up on history starting around 1900. I also had a huge interest in this exam, and love to spend time watching shows on the Military Channel on the subject, but did study for it back when I took it. As an interesting aside I was in Paris at the same time of the 50th anniversary of the defeat at Dien Bien Phu and saw a solemn ceremony at the Arc de Triomphe with some old veterans. This was probably a couple of months after I took the exam so the dates were still fresh in my mind. A second irony is that my wife and her sister have the same names as two of the fortified positions at Dien Bien Phu.
Here is a good book to prepare. A little over 300 pages. Where the Domino Fell: America and Vietnam, 1945 to 1995 Author: James S. Olson, Randy Roberts
There's a list of good study resources for the DSST Vietnam exam located here: http://www.challengeit.ca/2009/10/dsst-history-of-vietnam-war.html
I recently took the exam and passed. Wikipedia has some helpful information. Be warned that some questions deal with the aftermath, be it domestically in the US (all the way up until Bush) or the wars between Vietnam and the PRC and Khmer Rouge. One question on prehistory was easy since I just picked the name I didn't recognize from works on Vietnam (a wiped-out tribe).