To get a high vocabulary score, you should try to use a good range of vocabulary. However, this doesnt mean that strange or difficult words are the secret to success.
The vocabulary that you use also needs to be natural and appropriate to the style of the essay or report that you are writing. This is the reason why I suggested avoiding words like plummet and rocket in writing task 1. These words are not normally used in academic-style reports (because they exaggerate too much), whereas fall and increase significantly seem much more natural and appropriate.