In English, you focus on having good ideas and expressing them clearly and concisely.
A bigger vocab can be good as long as you have full mastery over all the words in your inventory. The questions you should pose to yourself when writing essays is - 'How can I make this clearer?', 'How can I make this shorter?' and 'Does this sound better if I use different words?'
The upside of a bigger vocabulary is that you will be able to express more ideas more effectively. A simple example: 'Macbeth, the tragic hero after which the play was named...' vs 'Macbeth, the eponymous tragic hero..'
A bigger vocabulary can be your downfall if you can't fully master the words you are using. So by all means learn more words and how to apply them while you're in study-mode, but when doing exams, always opt for the words you understand better to prevent yourself from sounding like a show-off idiot.
How many essays you should write:
To me, essay practice is for practicing to express your ideas effectively. It's for developing technique if you don't already have any.
There are two components that you need for a high-scoring exam essay.
1. Awesome ideas
2. Awesome technique for expressing them.
If your technique is already top-notch, there isn't much point to writing too many essays, unless you're keeping yourself from getting rusty. Instead you should focus on accumulating ideas.
Examiners want to see that your active mind is breeding interesting, original concepts. They get pretty bored after reading the same thing a billion times. Good ideas that make them think will stand out to them.
That said, no one's technique is really 100% perfect at the start of VCE, so aim to write at least 1 essay a week with the intention of challenging yourself with using different formats and different vocabulary each time.
By the time you reach the last part of semester 2, you should have a comfortable 'set' of sentence-structures and a good enough vocabulary that you can call your own set of tools.
Happy English-ing