I guess one danger of a gap year is that you might never go back to uni at all, but I somehow doubt that would happen to you.
I can't see this happening to you either, monokekie... I took six months off, and I felt my brain turning to mush... I started to crave uni and learning and such, and I think you'll be much the same. Instead of a gap putting me off of going to uni, it did the complete opposite and convinced me that it was the best thing for me to do since I was able to step back from it all and take a good look.
Dw about forgetting vce stuff... a lot of it really isn't related and/or is retaught at uni... if you're really that worried about it you can always spend a couple of weeks doing some basic revision just before you head to uni... the only subject I recommend you keep up is Chinese, because language competency is difficult to maintain without constant practice.
I think if you have the chance to travel with your parents, you should jump at the opportunity - there's just so much out there and you can benefit so much from seeing it all early in your life.