@acciodraco
I think if you put your mind to it and work hard over the break, you could definitely improve your Japanese level. I've always enjoyed studying Japanese, so I don't really know how to make one start enjoy learning Japanese and become motivated. BUT if you can find something in Japanese culture that makes you want to learn Japanese, that could become your motivation (like for me)
For learning vocab, first I read over vocab in my textbook. Then when those words would pop up in drama, music or anime, they would stick in my memory.
For writing, you need to understand the VCE grammar really well. Watching drama, listening to music also helped me in this area. Even if you just always read English Subtitles, the fact that you are listening to Japanese helps your comprehension and understanding of the language. To improve your writing itself, you could try use something like lang8 (
http://lang-8.com/) I didn't really use it myself but it looks like a great forum to improve your writing skills. Basically it's an exchange off, you correct other people's English, Japanese people will correct your Japanese.
For speaking, I had a tutor with whom I had conversation practice on a regular basis (nearly weekly during year 12). To improve speaking, a tutor is the best method as they can point our your mistakes. You could also try to speak with a native or friends in Japanese as much as you can to help you improve. I've never been but I have friends who go to Language Exchanges to help meet other Japanese people and people learning Japanese.
http://www.languageconnection.org.au/language-exchanges/ That could also be a good forum to get some speaking practice.
During Year 12, I would always look forward to Japanese class because it seemed like a break from all my other classes (not in the way that we didn't study, but because I enjoyed studying Japanese). Our teacher was really good so everyone always paid attention in class. I was probably lucky to be in that type of situation.
In your case, you still have plenty of time til next year. If you build your vocab and your Japanese ability gradually, you'll start seeing results by the end.
Good luck, it can look daunting but Japanese can be a fun subject to study for (unlike other VCE subjects). It might be a bit more challenging to study for as you can't cram all the work right before the SAC/exam, but with gradual work you can do well.