I flew VN years ago but with a VN ticket (really great BC fare LHR-Australia). As I recall the VN www sit is really picky about accepting even their own PNR. Needed to be case sensitive and my name had to be entered exactly as ticketed or maybe all UC.
I seem to recall being able to lookup a booking with the ticket number, too, but my ticket was on 738 stock.
I did manage to call their SF office but long hold times and west coast business hours; once I got through they were very helpful.
If you have a voip phone service, maybe try calling VN's Australia phone # - OZ-VN is a pretty active market with VN flights to both SYD and MEL.

Also, I seem to recall an FT post about finding ST PNRs using the web sites of some of the 'OTHER' ST airlines, as I recall Garuda and Saudia although I just tried to lookup a DL ticketed VS PNR on Garuda and wasn't successful so maybe that doesn't work any more. It did work in the past as I recall.