VCAA can be difficult because they test new types of questions and applications of concepts not seen previously in any trial exams. Practice exams on the other hand tend to be harder due to copying the previous year's VCAA exam and then applying terrible wording, ambiguity, supposedly assumed material which is actually outside of the syllabus and so on to make it more difficult. And that's the reason why VCAA exams eventually seem easy when you do them last - it's because every practice exam just tends to base their exam off the previous years' VCAA one, so you've practically already done the VCAA ones ten times over before you actually do it. And this is why every year, students will inevitably say that their exam was the hardest ever since new material will inevitably pop up that they have not seen before. So no, there's no assurance at all really.