Hi Goldorak,
Thanks for your reply, I agree with you, the K6 may be the 'valid' flights! LOL.
It is all a bit frustrating, as a sanity check I tried to test it by attempting to duplicate the search on Expedia, but KOS (sihanoukville) does not even come up as a search option ex SGN. Only KOS... the one in Greece! Strange.
I am also a bit disappointed at the whole booking process, I am doing multiple flights and I wanted keep everything in the one ticket/PNR. I am coming ex SYD and I am doing J class where available. I know that VN/K6 does not operate J class on some routes, SGN-KOS for instance (its a small ATR aircraft), so when I did my search on VN website, SYD-SGN-KOS and put Business class in, the page gave back an error. I would expect it to book it all in, but obviously give a message like so many other airline websites do; "Requested booking class not available on SGN-KOS" etc. In the end I ended up booking separate tickets.

Booked all my flights eventually which is great... now I just need to find out the timings of those flights!