Wouldn't it in fact be advantageous through VSL in that sense because you have contact with more people you don't really know? In school, you would know your teachers well, so you'd still have to find an external source. What sort of person do you recommend? A different teacher, tutor, ex student, native student/pen pal?
It's so not much how much you 'know' your teachers e.g. on a personal level, but other aspects also. The question and interview style varies from teacher to teacher. The words they use, their voice, their speed - there are so many things that will vary so widely outside the one or two teachers you've practised with. Becoming too comfortable with the teacher you're with is also not beneficial as it won't prepare you for the anxiety of speaking in front of new people. So, no, VSL is not advantageous at such because there would be fewer teachers to practise with and you would get too used to their style and habits or whatever.
As for how to fix the problem - I think it really does depend on your networking. Our teacher really pulled in all of their connections and I practised with 11 different people (5 teachers (3 native 2 not), 2 student teachers (one native one not, 3 native speakers) many times. One was from my own connections and the trial with them was the worst of them all. Why? Because even though they're native, smart and very helpful, they're just not familiar with the system. Teachers (esp. native) understand how the interview works. They may have even been assessors in the past. They know what kind of questions will be asked, how hard they will pitch them, how to adjust to the situation, how to keep you calm and how to draw out your skill. A native speaker with no connection to the VCE will have no idea about those kind of parameters. For example, my friend asked me something like "日本に行ったら、(あなた)も一人”になってしまうかもしれません。だとしたら、どうやって人を働きかけるか少し説明していただきませんか?” (When you go to Japan, you may also be alone. If such a thing were to happen, would you please explain to me how you would go about drawing people to you [to become friends]?)
You can guess I was never asked that in the exam. *としたら (if it were...) *~ていただく (more polite version of ~てもらう) are both not VCE-level grammar and even "~てもらえませんか" would be rare. 働きかける is also not VCE-level vocabulary.
If you do decide to do Japanese no matter what, go for VCE Japanese teachers. Any and all at your school and any and all the teachers they know. Past students - unless studying Japanese post-VCE (but even then...) - are a plausible option but they only have their own experience to go off which probably is not relevant to you. Pen-pals will teach you Japanese, not VCE Japanese.
One of the biggest things I learnt this year is how VCE Japanese ≠ Japanese. I estimate there would have been many proficient and skilled Japanese speakers who will be underestimated thanks to VCE. You're assessed on how well you use basic-level Japanese instead of how wide your knowledge of Japanese as a language is. It doesn't take into account your fluency - just your understanding, or how much you scour your work for minor mistakes. In real life, Japanese speakers don't take points off for how much grammar you use in your sentences or how advanced your vocabulary is.
It sounds like I'm very bitter (I have no reason to be - I did enjoy Japanese and went well (hopefully)) but if you want to learn Japanese because you love it - the answer is not VCE... in my opinion. I certainly won't be repeating it unless definitely necessary.
Anyway, sorry for ranting but there's more of my opinion lol