What do you mean by vocabulary? Do you mean that there are words in reading comprehension passages that you don't understand?

One technique you might want to try is looking up every word you don't know, then making a flash card for that word, with the word on one side and the definition on the other. That way, you can review all the words you missed.

I suggest keeping a Vocab Journal. Rather than writing down definitions, create it as you would a thesaurus. Add a general word to each page, "Happy", "Sad", "Anger", "Truth", etc.

When you learn a new word or see a word you don't know if your GMAT studies, add it to the page that has the closest essence to its meaning. Try to use a different color pen for each page. Every day, repeat the list for each page.

In this way, you are connecting words to other words with similar meanings, and not focusing on memorizing by rote. This is the "Word Groups" method and its my favorite. Good luck!