Tinh Van
This Vietnamese site offers a free English-Vietnamese dictionary available online. Translations include phrases using the requested word, as well as similar words if a requested word is not found.
http://www.tinhvan.com.vn/scripts/tvis/webdict/webdict.pl

The Free Vietnamese Dictionary Project
This website of the University of Leipzig in Germany is home to an English-Vietnamese dictionary that can be searched free online or downloaded. Vietnamese words are also listed in the Roman alphabet.
http://www.informatik.uni-leipzig.de/~duc/Dict/

CleverTranslate
CleverTranslate offers free and unlimited translation services. It provides instant translations for words in English into Vietnamese.
http://www.clevertranslate.com/dict/translate.jsp

Logos
Logos bills itself as the largest freely accessible multilingual dictionary on the web, with dictionaries in 232 languages. English words and phrases can be translated into Vietnamese at the touch of a button. Logos also features a universal conjugator, children’s dictionary, and more than 43,000 online books in over 100 languages, including 156 in Vietnamese.
http://www.logos.it/

Dicts
This English-Vietnamese dictionary translates words from both languages instantly, and allows the user to type in just part of a word to find all possible matches. The site also features a picture dictionary.
http://dicts.info/ud.php?l1=Vietnamese&l2=

Ectaco
This site offers instant translations of English and Vietnamese words. This free English-Vietnamese dictionary contains 400,000 words.
http://www.ectaco.co.uk/English-Vietnamese-Dictionary/