Your pieces (possibly with the exception of context) have to be of an academic standard, so if your vocabulary is too basic, you will be likely to lose marks for being unable to express yourself in such a manner. However, this doesn't mean you need to type like
pooshwaltzer in order to score well. As you can see from that example, it's often actually detrimental and I usually advise my less naturally talented students to just stay at a basic, academic level of vocabulary and not attempt to expand it because more often than not, it becomes detrimental to their marks in the process. While a complex vocabulary in the right hands can be quite effective, if you don't already possess one, I wouldn't go about trying to develop it now because there might not be enough time to develop your expression with them. Even if there was enough time, it'd be much better spent developing your ideas, learning your structure and developing your expression with the vocabulary you already have. In the end, there's more to English than just expression and content is a large part of it. I'd focus more of my time there. I go into detail regarding expression in my Summer prep course found
here if you want help developing it into a more academic level though.