What 'extra' work should I be doing outside of class they'll be able to expand and improve my vocab and sentence structure? I was planning on reading well written essays and expand my vocabulary off those since I thought it'd be more applicable when VCE comes, should I scrap that and just expand my vocabulary off reading books?
You can definitely use Year 12 essays, but don't limit yourself to them. I can't speak for everyone, but I know my expression and vocab didn't just come from reading past students' work. There is an element of self-study and independent investigation involved with acquiring words, and it's always more helpful to discover your own than rote-learn lists that other people provide.
For a word to be 'good' in an essay, it usually has to be accomplishing something another (simpler) word can't. You can't just use sophisticated language without sophisticated ideas, which is where a lot of students go wrong; throwing around words like 'bildungsroman' and 'preternaturally' because they think they sound clever, when in fact such language stands out a mile away as being pre-planned and clunky, especially if the word is out of context.
Good language is
efficient language, enabling you to cover concepts quickly and comprehensively; there may be helpful lists of generic 'good' words, but in the end the most helpful ones will be tailored to your writing style.
For instance, a friend of mine used to use the word 'depicts' ridiculously often in Language Analyses. Our class was all given the same 'good language' worksheets and handouts, but what he really needed was just a long list of synonyms and alternate ways of saying 'the author depicts >something.<'
Only by writing, revisiting, and seeking feedback on essays can you locate these areas of weakness.
But as ever, the more words you have at your disposal, the better. I'm yet to hear anyone come our of the English exam complaining that they knew
too many words.
I'm interested in true stories specifically from the WW2 era, I don't think they'll be much help though? Can you vouch for any books around the Year10/11 level, I'm interested in war/adventure genre.
Since you're in Year 10, I can't yet comment on just how useful this genre will be, because there's a Study Design change happening for your year level that they haven't fully explained yet. But in terms of whether they'll be helpful for improving general skills, definitely!
I bought a book the other day called
'We Will All Go Down Fighting To The End' which is a collection of Winston Churchill speeches, and am loving it. So you could look for similar books that explore the characters and personalities behind famous events, or even (auto-)biographies about such people.
Another one that I recommend to a lot of people is the
'Dangerous Book of Heroes' (from the
'Dangerous Book for Boys/Girls' series, if anyone remembers them?) It's a big collections of stories throughout history of people who have performed some act or acts of bravery, covering everyone from Arctic explorers sacrificing themselves for the chance that one member of their expedition team might survive, to the Bletchley Park geniuses who saved thousands of lives, to a primary school teacher who gave her life shielding five/six/seven year-olds from a school massacre. Some of them are very adventure-based, plot-driven stories, and others are quiet acts of determinism and strength that are wonderful to read about.
Or, if it's specifically WWII that you're interested in, then consider some history magazines or online journals (
History Extra from the BBC and
the Smithsonian mag come to mind as good ones.)
About writing words I learnt in a diary, how should I go about it? I fear that I'll write and try to remember too many words and end up forgetting them or not being able to apply them. I experience this when I learn new words in Chinese...sometimes 
Either at the back of your current English exercise book (do people still use them? Does every school have i-pads now? idk) or just in a separate little notebook or something, keep track of all the words you find, ideally with a sample sentence or definition next to it. Some students do this on their phone as well (on whatever note-taking app you have) since it means you can scroll through it on the bus or while waiting for a friend to text you back or w/e. Basically you want your word bank to be something at your fingertips, rather than just tucked away in an English folder somewhere, since exposure is the only way to make the new stuff sink in.
You will forget words. Constantly. But let's say you accumulate 300 words over the course of this year. At the most conservative estimate, you only truly learn and feel confident with 10% of them. That's still 30 whole words you didn't know before. Maybe one of those 30 will be in a prompt or article you get on an assessment piece, or maybe one of those words is
exactly what you need to describe a character. 100% retention isn't necessary for vocab expansion to be a helpful exercise
