Vocabulary can also improve naturally, assuming you are exposing yourself to various fields and that these fields actually fit you (words will naturally be absorbed). Of course, some people are fit for learning vocabulary for the sake of learning vocabulary. Others never accumulate a huge vocabulary (or even have a very simple one), but they excel in using what they have. In fact, you don't want to have a huge vocabulary, but nothing worthwhile to actually say. So, vocabulary is not something that one can necessarily force into himself and is accumulated over time from various sources. Syntax can be learned methodically or can also be intuitive. In fact, I believe syntax and intuition might be inseparably linked.