Can I do this itinerary for 72 hour TWOV: SGN - BKK - PEK - SGN

Yeah, this route is ok for 72h visa exemption. Enjoy your two days visa-free trip in Beijing!

Since I have a connecting flight SGN-BKK-PEK is this still counted as leaving from Thailand (not from Vietnam) as I'm coming back to Vietnam. Of course when I arrive in PEK I will only show the ticket from BKK-PEK and they will see I return to SGN ...but I need to check in and the airline will check its OK before they allow me to check in and go to China, will the airline accept me to check in for this? ..... thanks

Yes, the airline will allow you to check in. Don't worry!

Thanks very much Dylan for your reassurance!!