Kiwi.com are IMO the gutter of the 3rd party travel agents. They stitch together what appear to be connected inteneries that many might assume are connecting tickets offering connection protection, but are anything but. They have improved their messaging slightly pointing out that you need to go airside but they are still economical with the truth. Did you read the terms and conditions?

When you book a self-connect you need to understand that if your VietJet flight is late, and you can't exit the airport and check back in for JetStar in time, you're on your own. Remember both airlines are low cost carriers so you can't expect any compassion if you arrive late for check in, hand luggage too big, etc. If kiwi do offer any assistance in this situation they are incredibly hard to deal with, their customer service is typically shocking and you just need to search online for tales of woe. Many in this situation would suggest building in a multiple hour gap between the two separate flight (at least 6) or an overnight stay, and consider what your options might be if your first VietJet flight is delayed or cancelled at short notice, what options would you have to get yourself at short notice to SGN (assuming there are any seats left) so you can check in for your onward flight to SYD? Or do you just abandon going via SGN and now buy a new lastminute ticket to SYD from SIN?

I am sure many people manage to fly successfully with kiwi booked tickets, but when their self-connects go wrong the consequences dawn on the passenger and they usually face a very frustrating and worrying situation end up paying many more times than their original ticket cost to remedy the situation and recover their travel plans often with no assistance from kiwi at some random transit point they are not familiar with. YMMV.

If I recall you were previously asking about skiplagging an AirAsia ticket at KUL so your penchant for doing things a bit off piste and probably appetite risk is up there, but you need to do these things with your eyes open. At least you do come on here and ask first!