visa enquiry

Hi there, I am enquiry about my visa I am a South Korean citizen. I will be visiting Vietnam from 10th November till 22nd November 2009, then I am going to Cambodia by a bus for a week then coming back to Vietnam for flight to Melbourne, but I only purchased airplane tickets for Korea to Vietnam(Hochimin city) on 10th Nov and Vietnam(Hochimin city) to Melbourne on 30th Nov. I know as a Korean citizen I am able to visit Vietnam up to 15days but my airplane ticket shows I will be in Vietnam for 21days, not shows that I will be visiting Cambodia. In this case, do I need visa? Or do I still need to prove that I will be visiting other countries by purchasing bus or train tickets? Or it’s okay as long as I have return ticket? I look forward to hearing from you soon. Thank you.