As a new student of Vietnamese, the two compact dictionaries recommended for the class were inadequate, even for entry-level study of Vietnamese. Both were very compact, bi-directional, and seemed potentially useful for tourists, but with only about 15,000 entries apiece, many simple/common words were missing. I needed something more comprehensive than these, but Tuttle's reference dictionaries were also too large for everyday carry and were NOT bi-directional (2 volumes, one for Viet->Eng and one for Eng->Viet).
The Langenscheidt Pocket Dictionary has been the perfect compromise for me: semi-compact dimensions, a more substantial collection of words (40,000 entries), and bi-directionality.
As a (very) thoughtful design touch, the cover is made of a protective vinyl instead of paper. The paper-back covers on my other two dictionaries required me to take extra care in taking them in and out of my bag to prevent excessive wear, but the more robust cover on the Langenscheidt dictionary obviated the need for such careful handling. While I certainly wouldn't carry this in a pants pocket (too thick), for MY needs and purposes this dictionary is small enough, sturdy enough, and comprehensive enough to go wherever I go. Unlike the travel dictionaries previously mentioned, this one does NOT contain specialized sections with useful phrases for travelers. If you are only traveling to Vietnam for a short visit, you might also require a phrasebook to get you by.
DISCLAIMER: I have had this dictionary less than a month, and I am only a few months into learning Vietnamese, so I cannot comment intelligently on the accuracy of the translations/definitions or the relevancy of the words they chose. For what it's worth, the previous edition, Langenscheidt's Pocket Dictionary Vietnamese/ English, English, Vietnamese (2001), has also received positive reviews from many others.