Thanks for your comments.
The Reading test is the real test of vocabulary in IELTS. You have 1 hour to read 2500 quite difficult words. So is the Speaking test: you are expected to use vocabulary naturally to discuss abstract topics at an advanced level.
Task 2 in the IELTS Writing test is NOT a vocabulary test.
Let's look at the official descriptors for IELTS Task 2 Writing (public version) here, under Lexical Resource.
First of all, vocabulary, or 'lexical resource' as IELTS calls it, is only 25% of the final band. Developing and organizing your ideas, being grammatically accurate account for 75% of the marks.
Secondly, there is no real expectation that you need to use 'advanced' vocabulary.
- Band 9: uses a wide range of vocabulary with very natural and sophisticated control of lexical features
- Band 7: uses a sufficient range of vocabulary to allow some flexibility and precision; uses less common lexical items with some awareness of style and collocation
- Band 6: uses an adequate range of vocabulary for the task; attempts to use less common vocabulary but with some inaccuracy
- Band 5: uses a limited range of vocabulary, but this is minimally adequate for the task
- Band 4: uses only basic vocabulary which may be used repetitively or which may be inappropriate
The phrase that recurs here is 'range of vocabulary.' A range means that a person in Band 9 may use some unusual words but may also use very very simple and ordinary words. And when the Band 9 writer uses an unusual word, it is used absolutely appropriately: not because it is in a list of 'difficult' words somewhere, but because it is the best word to express the idea of the sentence.
There is no IELTS list of 'words-you-must-know-for-Band-Eight.' Examiners do not have a checklist of words to distinguish between Band 6 and Band 9, or between Band 6 and Band 6.5.
There is no need to use 'sesquipedalian' when 'long' would do: it's about choosing the right word for the right situation, not siimply dropping fancy words into your essay at random.
I like the phrase in Coherence and Cohesion: "in such a way that it attracts no attention." This should be our aim in vocabulary as well: not to draw attention, but to use the right word for the right situation.
Task 2 is not a formal paper or a term report. It's not a scientific paper or an international treaty. Equally, it's not an SMS message or a movie dialog or a record of a conversation. It's somewhere in between in terms of formality and structure. Using too formal or too informal a structure or a vocabulary item in an inappropriate way will lower the band.
So avoid any temptation to show off in writing. Your essay should be a perfectly cooked dish, with no ingredient (think vocab item or grammatical structure) overpowering any of the other ones.
Don't have complicated words that stand out like sore thumbs. A rose in a cabbage patch is a weed. Instead, think of the most suitable and natural word to express your ideas.
Avoid passives, avoid not only/but also, avoid over-formal 'To illustrate' or 'Verily' or 'In the following report I shall endeavour to outline the chief contributory factors'.
Just write a nice, simple essay with nice, simple ideas. It's not for nothing that the first criteria on the left in the descriptors is Task Response, followed by Coherence and Cohesion, and then Lexical Resource. Content is king.
Here's what Mark Twain, the American writer, journalist, and traveler who gave us Huckleberry Finn and Tom Sawyer had to say about long and complicated words:
...an average English word is four letters and a half. By hard, honest labor I've dug all the large words out of my vocabulary and shaved it down till the average is three letters and a half.
...I never write 'metropolis' for seven cents, because I can get the same money for 'city.' I never write 'policeman,' because I can get the same price for 'cop.' And so on and so on. I never write 'valetudinarian' at all, for not even hunger and wretchedness can humble me to the point where I will do a word like that for seven cents; I wouldn't do it for fifteen.
Mark Twain: Address at the annual dinner of the Associated Press, at the Waldorf-Astoria, New York, September 18, 1906.