There is no specific vocabulary section in the IELTS test but you will need to understand and be able to use a large number of words to do well in all sections of the exam.
If you are doing the Academic version of IELTS you should study the words in the Academic word list.
Vocabulary learning tips
Read as much as possible. If you come across a word you don't know, write it down or look it up.
Use a dictionary. Many browsers include dictionaries.If you are reading online, you may be able to right-click on a word to look it up.
Sign up for a 'word a day' email.
Think of ways of remembering words. Do they sound like a word in your own language?
Remember words are often used together with other words. Try to learn these 'collocations.