Duolingo.com has now an English Course for Vietnamese People. It's like a game and it's free to use.

Memrise.com is also very good and free. It works with pictures and written sentences around the new learned word. This way you can remember the vocabulary quicker and forever.

There are some ideas on a similar discussion:

http://www.italki.com/discussion/79890

Everytime i encounter an unfamiliar word, i write it down on my notebook and search its meaning in the dictionary. I study how the word is used in the sentence and make some sentences using that word.
:) 

Before I would try to learn vocabulary from a list. It never worked for me. Now, I realize that SRS works best for me. I use flashcards or systems like Anki. With Anki I learned about a hundred new words a week! You should give it a try and see if it works for you!

   There is a basic format for acquiring  vocabulary.

#1  Read it.  #2  Write it.  #3  Say it.

Otherwise,  one can begin with the Dolch Lists for the most common

Verbs and most common Nouns in the English language. Make sure you know them first.

Otherwise,   try to focus your studies in a specific area, such as Science, Technology,  Math,  Engineering,  Art, History,  and so forth.

   Reading Non-Fiction is better than Non-Fiction for the acquisition of an advanced or academic vocabulary.  Otherwise,  if one's  familiarity with English is very modest,  Children's stories are an excellent study  material.

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