Is there a correlation between vocabulary and IQ?

Answer 1

Yes. A large vocabulary is indicative of high intelligence. This is not to say that a large vocabulary increases your intelligence; rather, it's to say that a high intelligence is what enables you to acquire a large vocabulary. Hence the correlation between vocabulary and IQ.

Some people try to artificially inflate their IQs by memorizing lists of esoteric words; however, this has very little, if any, positive effect on increasing their intelligence. As you acquire more and more words, the differences in word meanings become more and more subtle (e.g. connive and collude, although similar, have a shade of difference in their meanings); the ability to distinguish between shades of meaning in words requires a keen intellect.

People who try to artificially expand their vocabulary size end up misusing the words they've memorized because they simply lack the intelligence to grasp the subtelties of language.