Hello!
I would highly suggest buying one of the VCD textbooks - there are a few of them, and read them during the school holidays to get the ideas and concepts fro units 1 and 2.
VCD requires a lot of terms and things to remember, I'd mainly suggest looking at design fields, the design process, principles and elements (if you've done studio arts or other arts subjects they're pretty much the same), as well as technical drawing (mainly orthogonals, as they have a LOT of rules).
I can give you a quick run down of what the course is like - first of all, you'll do a few small SAC and SAT tasks, and then a folio that is due in September. You can do your folio on anything you want, usually people tend to do logo designs, however it needs to be based in communication, environmental and/or industrial design. For example, my folio was to fit the brief of creating a cartoon cat character and a book cover for it. It requires 2 final presentations. I'd recommend finding other VCD students and looking at their folios. I can send you some photos of my folio once I get it back, if you'd like. I also recommend going to top designs to get inspiration!!
Overall I really enjoyed VCD. However, be wary of the exam, as its only one hour and a half and you have a LOT to do in the exam for that short time frame.
The main thing I struggle with is the technical drawing as I'm a more free based art student, but if you can get the basics of technical drawing down pat, you'll be fine.
You WILL need to study some units 1/2 stuff to get a good score for your SACs/SATs, but its possible and isn't too hard to do!