I'm with Russ - I personally reckon you're better off accepting your offer, doing a full year at uni, and using those marks to get a good transfer into your desired course. I know that I, for one, improved my marks upon entering university, because the environment made me a lot more motivated than any high school could hope to achieve. In fact, I could've very easily moved into many different courses that weren't available to me by my ATAR with the WAM (overall uni mark) that I achieved last year.
Unless you wanted to work/travel during your gap year, and you're only taking the extra VCE subject to give yourself some more chance, as opposed to repeating VCE being the main focus. Although, if this is the case, you may as well just apply for VTAC next year - you don't HAVE to apply in the year you graduate.