Plus CX flights seem to be notoriously late into SGN for some reason.

Does anyone know what the MCT is for I/I connection at SGN? I'll be flying in from BKK to SGN and then onwards to DOH on the recent QR deal...

I don't know what the "legal" MCT time is but you should be able to do it in 10 minutes worst case.

I understand you will be travelling on separate tickets. If yes, the notion of MCT is irrelevant in that case. Even if you are compliant with the MCT, you have no protection in case of misconnection.

What about a situation where a QR PNH-SGN flight is followed by QR SGN-DOH on separate tix.

How would this be handled? Would a BP be issued for the onward flight in PNH? Would I need to deplane, go to the transit desk and then reboard? At least I'd be assured of not misconnecting.

I don't know if the 2nd BP will be issued in PNH. Could be but could be not. If not, you will have to collect it at a transfer desk (if any).
Another issue to consider when connecting in Vietnam on separate tickets is that many citizenships require a visa to enter Vietnam and if you don't have one (and require one), the airlines should logically deny you boarding the 1st flight (to Vietnam) because in the booking (in their system), you just go to Vietnam. Showing an e-ticket for an onward flight is sometimes not enough. In your case (QR to QR), it might be easier to convince them as they can see everything in their system and they can even create a kind of link between the 2 PNRs

The visa scenario is easily handled by procuring an evisa ahead of time. The $12 (or whatever) it costs would be worthwhile even if it's never used.

The quick turn in SGN is the concern if the BP isn't in hand.

SGN does not see many int'l to int'l connections. However there is a transit desk in the main immigration area. It is usually staffed by just a few people, I believe they are VN staff. I've never seen other airline staff there, but there have been some people mention they transited ok. I saw, I think in the VOA thread, one comment saying the staff go out and get your boarding pass for you, bring it back into the airside area, and you can transit int'l to int'l. As with all things Vietnam, YMMV. If you do have a bp already, I'd just approach that desk and ask them where to go, cuz it isn't clearly marked that I know of where there is a stairway or elevator and security to get upstairs.

If you have an evisa and are willing to go through immigration, then your only question is about the length of lines at immigration, and those vary. I've been through many times, some only a few minutes, and others at the "rush hour" of many flights landing it took 30-45 minutes. Departure lines have also been pretty lengthy at times, but the newly remodeled departure immigration and security area has kept that moving quickly as of late, even when the lines are a bit long. Basically my point is that if you are going to go via immigration and going upstairs, it's again YMMV but biggest factor might be if the QR flight lands at the same time as some other widebodies.

Thanks for this info. My flight isn't for a few months yet so plenty of time make a decision. ^

I'm considering doing PNH-SGN before my SGN-DOH flight, so naturally I came across this thread. I tried pricing the itinerary out as separate segments on ITA and could only get 25 hour connection (i.e. it didn't work like I was hoping it would with a ~1hr connection).