Google Translate, SDL Free Translation and Nice Translator are three online tools for translating phrases from English to Vietnamese. Google Translate also has apps for both iOS and Android users, while Bing Translator is the translation engine embedded into Windows Phone.
Google Translate is ubiquitous. It is free, web-accessible and mobile accessible for both iOS and Android. It works well with all browsers and is the engine behind Chrome's auto-translation feature. It also has an audio component which is helpful for learning Vietnamese.
Both SDL Free Translation and Nice Translator are free to use on their websites, but they don't have mobile versions.
Bing Translator is an embedded translation engine for Windows Phone, just like Universal Translator on MacOS. Users need neither to pay for them nor to take time installing them.