I don't know when you take your CET-6. If the test is coming near, I don't think it possible to enlarge your vocabulary in a short time.
I had TEM-4 last year, which was a little harder than CET-6. I think the only way to get a better mark is to do simulating tests. Make sure you are clear about the answer, especially why the right answer is right, and why the wrong answers are wrong.
Enlarging your vocabulary is a process of accumulating. There are three kinds of words: strang words, inactive words and active words. Strang words mean the words that you have no idea what the words are about. Inactive words mean the words you know a part of parts of the meanings of the them, but you can't use it freely without mistakes. Active words mean the words that you have completely mastered, and you know every part of the meaning of the word.
In fact, the vocabulary of CET-6 requires you to master 2000 most frequently-used active words and maybe 3000 inactive words. So what you need is to enlarge your inactive vocabulary (although I don't suggest doing so).
I guess you are not an English major, so you don't have enough time for everyday English study. As an English major, I almost study English the whole day every day. So to me, vocabulary is not a problem, because I listen, speak, read and write a lot every day. Among these four elements, I think reading is the most important. So if you want to genuinely know more words, you have to read and read and read and read and read... Besides, you have to make sure you know the word's grammartical feature, context, morphology, collocation, synonyms and other aspects, plus how it is different from its synonyms.
To be a good master of English is very hard, but once there is a will, there is a way which is practice and practice and practice and practice and...
p.s. : I am about to take TEM-8 next month. So, do our best together. Good luck!