A wide vocabulary is important for all parts of the IELTS test.
For the speaking exam is essential to use some idiomatic expressions to get to 7 for vocabulary [lexical resource]. In order to achieve this I suggest you learn 10 or 20 common idiomatic expressions that can be used for a wide range of topics. The idea is to learn a few well rather than trying to learn many and not being able to use them correctly or remember to use them. Practice using them by looking at sample questions and thinking which idiomatic expression can be used.
The public version of the IELTS speaking test criteria
The public version of the IELTS writing test criteria task 1
The public version of the IELTS writing test criteria task 2