GRE® vocabulary questions fall into two categories: Text Completion questions, those in which you have to choose any of one, two, or three words to fill in a corresponding number of blanks; or Sentence Equivalence questions, those in which you have to fill in a single blank but must choose the two words that could fit into the sentence. What many students do not appreciate about these questions is that in both cases, context establishes what must be the correct response. That is, you could study several hundred vocabulary words, and there are online lists as well as books dedicated to the topic, but on the day, you might just get some other word you had not studied to potentially fill one of those blanks. Again, being a strong reader and picking up on contextual clues is a vital skill that should not be overlooked in any serious course of study. Another consideration: no matter how many words you may memorize, there will only be about twenty such blanks to fill in on the whole test (more if you get an experimental Verbal section, but that is ungraded). I would not discourage anyone from boning up on their vocabulary, but at the same time, a mature approach to the questions, combined with a fairly large vocabulary, will beat out a memorize-x-words-a-day approach any day.
Good luck with your studies.