I have flown VTE-PNH-SGN on VN a few years back and we deplaned at PNH and were escorted to a stairway back up to the departure level to re-embark on the same plane without any immigration formalities. However, that was all part of the intended plan for that particular route and there were several other passengers in our little "transit group" so they were obviously prepared for us to do just that. I kind of suspect doing a strait turnaround would be liable to confuse everyone at the airport and thus yield very unpredictable results.